The bug
While boats block the rendering of water as expected, they do not block seagrass from rendering. As a result, it can appear inside the boat.
This may be a parity issue by virtue of the existence of MCPE-139077, however testing is needed to confirm this.
How to reproduce
Locate seagrass or tall seagrass (the latter is preferred due to its height) directly at the surface of water
Place a boat on top
Look closely
Expected results
The boat would appear normally.
Actual results
Some pixels penetrate through the boat from the seagrass.
Can confirm in 1.19.