regarding the cheat codes, Gameshark and Action Replay (and their rereleases) are the same thing, just distributed under different names.
the only other one released to my knowledge was the codebreaker, which according to the segaretro page on it is compatible with the same codes.
I'll give it a shot next week and return with my results.
Either way, there's 19 US games that can't be tricked into working according to what I just read.
Hydro Thunder
not 100% accurate.
there's 2 versions of Hydro Thunder, the original release and the Hot! New!/Sega All Stars rerelease
you can tell by the color of the disc.
the OG release has a blue top to the disc and doesn't support VGA
the rerelease has a red disc and does support VGA, as well as having a couple bug fixes
buuuuuut maybe it's the other way around.
again, I'll test it out next week when I have some time.