diff --git a/src/stream/src/executor/hash_join.rs b/src/stream/src/executor/hash_join.rs
index bb6e1027c9a40..a30e25af95c5c 100644
--- a/src/stream/src/executor/hash_join.rs
+++ b/src/stream/src/executor/hash_join.rs
@@ -779,9 +779,10 @@ impl<K: HashKey, S: StateStore, const T: JoinTypePrimitive> HashJoinExecutor<K,
             .positions(|idx| *idx == watermark.col_idx);
         let mut watermarks_to_emit = vec![];
         for idx in wm_in_jk {
-            let buffers = self.watermark_buffers.entry(idx).or_insert_with(|| {
-                BufferedWatermarks::with_ids(vec![SideType::Left, SideType::Right])
-            });
+            let buffers = self
+                .watermark_buffers
+                .entry(idx)
+                .or_insert_with(|| BufferedWatermarks::with_ids([SideType::Left, SideType::Right]));
             if let Some(selected_watermark) = buffers.handle_watermark(side, watermark.clone()) {
                 let empty_indices = vec![];
                 let output_indices = side_update
diff --git a/src/stream/src/executor/watermark/mod.rs b/src/stream/src/executor/watermark/mod.rs
index 4d1b4b32beb6c..911b48e2b989e 100644
--- a/src/stream/src/executor/watermark/mod.rs
+++ b/src/stream/src/executor/watermark/mod.rs
@@ -18,35 +18,39 @@ use std::hash::Hash;
 
 use super::Watermark;
 
-#[derive(Default)]
+#[derive(Default, Debug)]
 pub(super) struct StagedWatermarks {
     in_heap: bool,
     staged: VecDeque<Watermark>,
 }
 
-pub(super) struct BufferedWatermarks<ID> {
+pub(super) struct BufferedWatermarks<Id> {
     /// We store the smallest watermark of each upstream, because the next watermark to emit is
     /// among them.
-    pub first_buffered_watermarks: BinaryHeap<Reverse<(Watermark, ID)>>,
+    pub first_buffered_watermarks: BinaryHeap<Reverse<(Watermark, Id)>>,
     /// We buffer other watermarks of each upstream. The next-to-smallest one will become the
     /// smallest when the smallest is emitted and be moved into heap.
-    pub other_buffered_watermarks: BTreeMap<ID, StagedWatermarks>,
+    pub other_buffered_watermarks: BTreeMap<Id, StagedWatermarks>,
 }
 
-impl<ID: Ord + Hash> BufferedWatermarks<ID> {
-    pub fn with_ids(buffer_ids: Vec<ID>) -> Self {
+impl<Id: Ord + Hash + std::fmt::Debug> BufferedWatermarks<Id> {
+    pub fn with_ids(buffer_ids: impl IntoIterator<Item = Id>) -> Self {
+        let other_buffered_watermarks: BTreeMap<_, _> = buffer_ids
+            .into_iter()
+            .map(|id| (id, Default::default()))
+            .collect();
+        let first_buffered_watermarks = BinaryHeap::with_capacity(other_buffered_watermarks.len());
+
         BufferedWatermarks {
-            first_buffered_watermarks: BinaryHeap::with_capacity(buffer_ids.len()),
-            other_buffered_watermarks: BTreeMap::from_iter(
-                buffer_ids.into_iter().map(|id| (id, Default::default())),
-            ),
+            first_buffered_watermarks,
+            other_buffered_watermarks,
         }
     }
 
-    pub fn add_buffers(&mut self, buffer_ids: Vec<ID>) {
+    pub fn add_buffers(&mut self, buffer_ids: impl IntoIterator<Item = Id>) {
         buffer_ids.into_iter().for_each(|id| {
             self.other_buffered_watermarks
-                .insert(id, Default::default())
+                .try_insert(id, Default::default())
                 .unwrap();
         });
     }
@@ -62,7 +66,7 @@ impl<ID: Ord + Hash> BufferedWatermarks<ID> {
 
     /// Handle a new watermark message. Optionally returns the watermark message to emit and the
     /// buffer id.
-    pub fn handle_watermark(&mut self, buffer_id: ID, watermark: Watermark) -> Option<Watermark> {
+    pub fn handle_watermark(&mut self, buffer_id: Id, watermark: Watermark) -> Option<Watermark> {
         // Note: The staged watermark buffer should be created before handling the watermark.
         let mut staged = self.other_buffered_watermarks.get_mut(&buffer_id).unwrap();
 
@@ -100,7 +104,7 @@ impl<ID: Ord + Hash> BufferedWatermarks<ID> {
     }
 
     /// Remove buffers and return watermark to emit.
-    pub fn remove_buffer(&mut self, buffer_ids_to_remove: HashSet<ID>) -> Option<Watermark> {
+    pub fn remove_buffer(&mut self, buffer_ids_to_remove: HashSet<Id>) -> Option<Watermark> {
         self.first_buffered_watermarks
             .retain(|Reverse((_, id))| !buffer_ids_to_remove.contains(id));
         self.other_buffered_watermarks
diff --git a/src/tests/simulation/src/main.rs b/src/tests/simulation/src/main.rs
index 3e4cf1a435f92..b6629bacb45fd 100644
--- a/src/tests/simulation/src/main.rs
+++ b/src/tests/simulation/src/main.rs
@@ -137,7 +137,6 @@ pub struct Args {
 #[cfg(madsim)]
 #[madsim::main]
 async fn main() {
-    use std::env;
     use std::sync::Arc;
 
     use risingwave_simulation::client::RisingWave;
diff --git a/src/tests/simulation/tests/it/nexmark_source.rs b/src/tests/simulation/tests/it/nexmark_source.rs
index cb50853a00500..9d1c784a8d8f6 100644
--- a/src/tests/simulation/tests/it/nexmark_source.rs
+++ b/src/tests/simulation/tests/it/nexmark_source.rs
@@ -21,15 +21,35 @@ use madsim::time::sleep;
 use risingwave_simulation::cluster::Configuration;
 use risingwave_simulation::ctl_ext::predicate::identity_contains;
 use risingwave_simulation::nexmark::{NexmarkCluster, THROUGHPUT};
-use risingwave_simulation::utils::AssertResult;
 
-/// Check that everything works well after scaling of source-related executor.
 #[madsim::test]
 async fn nexmark_source() -> Result<()> {
-    let events = 20 * THROUGHPUT;
+    nexmark_source_inner(false).await
+}
+
+#[madsim::test]
+async fn nexmark_source_with_watermark() -> Result<()> {
+    nexmark_source_inner(true).await
+}
+
+/// Check that everything works well after scaling of source-related executor.
+async fn nexmark_source_inner(watermark: bool) -> Result<()> {
+    let expected_events = 20 * THROUGHPUT;
+    let expected_events_range = if watermark {
+        // If there's watermark, we'll possibly get fewer events.
+        (0.99 * expected_events as f64) as usize..=expected_events
+    } else {
+        // If there's no watermark, we'll get exactly the expected number of events.
+        expected_events..=expected_events
+    };
 
-    let mut cluster =
-        NexmarkCluster::new(Configuration::for_scale(), 6, Some(events), false).await?;
+    let mut cluster = NexmarkCluster::new(
+        Configuration::for_scale(),
+        6,
+        Some(expected_events),
+        watermark,
+    )
+    .await?;
 
     // Materialize all sources so that we can also check whether the row id generator is working
     // correctly after scaling.
@@ -63,7 +83,7 @@ async fn nexmark_source() -> Result<()> {
     sleep(Duration::from_secs(30)).await;
 
     // Check the total number of events.
-    cluster
+    let result = cluster
         .run(
             r#"
 with count_p as (select count(*) count_p from materialized_person),
@@ -71,8 +91,15 @@ with count_p as (select count(*) count_p from materialized_person),
      count_b as (select count(*) count_b from materialized_bid)
 select count_p + count_a + count_b from count_p, count_a, count_b;"#,
         )
-        .await?
-        .assert_result_eq(events.to_string());
+        .await?;
+
+    let actual_events: usize = result.trim().parse()?;
+    assert!(
+        expected_events_range.contains(&actual_events),
+        "expected event num in {:?}, got {}",
+        expected_events_range,
+        actual_events
+    );
 
     Ok(())
 }