So, after a good night's sleep I had some time to think about all your suggetions.
I will continue working on the map in about 1 week, as besides my newfound profession of a junior mapper I sporadically have to study

.
Some of the things I am going to work on:
Water: The water shader looks absolutely great if reflections are turned on, but not if turned off (screenshot).
I will build a version with a shader and a version with a more realistic looking water texture, so you can decide yourself. Or I will need to create a completely new water shader... Anyway, I will work on that.
I will remove the trigger_hurt from the water and make it deeper, but only so much that the player cannot look below the water surface, in this way I do not have to add anything that worsens performance. I also will make it a pain in the a* to move around in (sorry

), so my original concept of game flow stays the same.
I will add some sharks (fins) and going near one becomes an instant kill.
Clips: I am going to work on the clips as well, trying to make it obvious where you can go and where not. I will keep the clips on the roofs. I will remove the trigger_hurt at the ceiling and experiment with something else.
I also will make the beach bigger to make it more inviting to play on. (And maybe a "sandier" texture)
Until then, some questions about the "story"/info_textures:
I am interested in who can and who cannot read the info_textures (letter on the desk, map at the wall, notes at the treasure chamber), due to quality/resolution settings.
Of course I am also interested in who found the treasure chamber based on the hints in the letter and on the map?
Thanks again for all your help.