noun
water level gauge; watermark
Refers to a physical marker or gauge that shows water level, such as on a riverbank, reservoir, or in a boiler. The English 'watermark' here means a mark indicating water level, not a digital watermark.
川岸の水位標を見ると、昨日より水位が上がっていた。
Looking at the water level gauge on the riverbank, the water level was higher than yesterday.
ダムの水位標は、貯水量を監視するために設置されている。
The water level gauge at the dam is installed to monitor the water storage.
標識 is a broader term for 'sign' or 'marker', whereas 水位標 is specifically for water level.
Compound of 水位 (water level) and 標 (mark, sign). The exact historical derivation is uncertain, but the word is a straightforward technical term.