Most of the details present in the Dreamcast version were sacrificed, replaced with only a few minor interface improvements.
Hydro Thunder is one of those games that prove that timing is everything. The game was rushed to the Dreamcast so that it could have the advantage of being one of the few reasonably good games available at launch, and it ended up being a beautiful, if not painfully exact, arcade port. Now, more than half a year later, the game has landed on the N64. While it's obvious the developers spent more time making this version of Hydro Thunder cleaner and more user friendly, without the graphical muscle of the arcade hardware or the Dreamcast, the N64 version simply isn't as impressive.
Aesthetics aside, the game hasn't changed at all from the Dreamcast version. You're still racing one of nine different boats across 12 different courses in an attempt to place first. Along the way you'll run into various obstacles, collect boosters, make insanely huge jumps, and do your best to keep your boat afloat. This game has only two modes - play on your own or play with a friend. There's no time trial, no tournament, no battle or practice mode, just the pay-per-race-styled "game" mode that you're already familiar with. Still, the user interface has been much improved, and it now sports a "restart race" command from the pause menu and offers a more comprehensive option menu. Unfortunately, the N64 version doesn't let you completely customize your controls and instead makes you pick one of two different preset control schemes, none of which emulates the controls of the Dreamcast version. This makes it almost impossible to pull off boost jumps, as the boost button is now nowhere near the brakes. Also the boats in this version seem to be almost weightless, as they float all over the screen at the slightest bump.
You'll be impressed by how well the graphics translate from the arcade onto the N64. While they're not perfect, the graphics, in general, look very nice. Unfortunately, a lot of the effects are missing, and sometimes the water looks plain awful. But the most noticeable problem is how badly the frame rate suffers, as you literally trudge along at points. What was once a fast and furious arcade racer has turned into a slow, dawdling trip through the courses, all of which seem much longer this time around.
Another department that suffered heavy losses is the audio department. While the in-game music is pretty much the same, the audio cues from your navigator and the engine noises are very low quality. Now all the boats pretty much sound the same, and the sound effects are rather generic.
While some areas of the game have been improved, the incredibly frustrating AI hasn't been. It's still the case that the computer-controlled opponents are faster or slower depending on which boat you select. If you choose a faster, more difficult boat to control, you'll be up against faster opponents. This makes it a bit lopsided, as you get none of the pros but all of the cons inherent in choosing a more difficult boat. So, in short, the best boats to select are the slowest ones, unless you're less concerned with winning the race than you are with placing the fastest time. Perhaps the most frustrating element of Hydro Thunder is the outright difficulty. You always start in 15th place, and the only way you can win a race is by hitting every boost, taking every shortcut, and hitting no walls. As if that weren't enough, Hydro Thunder retains the time gates - an arcade element. This makes finding shortcuts more accidental than intentional, as you've no time to stop and explore the track.
Most of the details present in the Dreamcast version were sacrificed, replaced with only a few minor interface improvements. While the N64 version is definitely prettier than its PlayStation counterpart, the sluggish racing keeps Hydro Thunder from being even half as much fun as it was in arcades, and the lack of other modes keeps the gameplay from being interesting.