Drive underwater in a moded USS Numnutz with just dff txd and handling line for the seaplane.
I did the handling about 18 years ago and made the DFF/COLL about two months ago. Some issuses sub can stall on heavy waves better not to pitch down water effects makes this bad on low end PC, and sub can stick to walls and go fast. Mass had to be low making it more will slow down sub, most any number change will have negative effects like fly.
Installation:
Use an Crazy IMG Editor to open gta3.img;
Make a backup of original skimmer.dff and skimmer.txd by extracting from archive;