This is a really nice 20KM SH5 water for SH3..water looks amazing.
http://www.subsim.com/radioroom/showthread.php?t=239064
M.E.P.S Version 5 with green water is also a good 20KM alternative
Rubini´s underwater dust&plancton effect v1 and Sh3 volumetric clouds&fog by Rubini v2.2 also improves the graphics.
These U boat skins are fantastic:
http://www.subsim.com/radioroom/showthread.php?t=214215
ANZACMICKS DAMAGE IMMERSION V0.5 open beta is a nice graphic damage improvement.
All of these are worth a look and improve the graphics in my opinion.
Kelly621