Turn shadows and AA off
also you may want to try the following
with sh running, press ctrl + alt + del. this will open the system manager
select the processes tab and left click SH5.exe
change the priorities to "high" or "real time"
resume your game
also, you may try this mod:
http://www.subsim.com/radioroom/showthread.php?t=163847
it changes the light in the game and (at least for me) reduces the impact on FPS in the interior of the boat.
Glad if I could be of any help
