Water from JB flowing into the sea will not raise sealevel. It might change salinity but not possible to raise sea level.
Here is a small extract on some study paper done -
Impacts of sea level rise in West Johor
Sea level anomalies in the Singapore Strait due to storm surges of the South China
83 In this study, we focus on extreme sea level anomalies (> 30cm) in the Singapore Strait
84 (SS) which are of great importance for the damages induced to the coastline, and we
85 relate them to large scale wind patterns over the entire SCS. We use the de-tided residual
86 heights from a long term (23.5 years) tidal record available at the Tanjor Pagar (TG)
87 station (Figure 1) as well as TOPEX/Poseidon satellite altimetry. It has become known
88 that the wind is one of the primary factors, if not the most critical one, responsible for
89 anomalous sea levels in the Singapore Strait as well as in other coastal areas of the world
90 ocean, like the North Sea and Adriatic Sea in the Mediterranean. The dominant wind
91 patterns over the SCS are the Northeast and Southwest (SW) monsoons which are
92 responsible for the strong seasonality of the atmospheric circulation over the SCS
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239 It has been noted that sea level dynamics in the Singapore Strait is the resultant of
240 combination of two major independent forces, astronomic and atmospheric; therefore an
241 extreme sea level might be constituted of the wind-induced surge having typical duration
242 from several days to a week, and an hour-long peak tide. The stronger the NE wind is,
243 and the closer the storm surge event to a spring tide, the higher is the resultant sea level in
244 SS. However, such extreme combinations are not frequent, and the majority of noticeable
245 cases are distributed somewhere between moderate storm surges during a spring tide and
246 strong storm surges during an intermediate tide. For example, SLAs corresponding to the
247 highest sea levels of the year (marked in the boxes), in Figure 6 (a, b & c) and Figure 7 (a,
248 b, c & d) are relatively smaller than other SLAs in the respective years, which indicate significant contribution of high tides. On the other hand, some
249 storm surges (even
250 extreme) occurring during neap tides can be unnoticed in tide gauge records.