CONTAINS ~~~~~~~/\~~~~~ SPOILERS
The first thing you need to know about this sequel is that it isn't Jaws. The second, and indeed the only other thing you need to know, is that it's not an absolutely terrible film that betrays, cheapens and degrades the first one. In some regards it's actually better, while in other regards (sadly, the most crucial ones) it just doesn't match up. How could it? Jaws is a classic, obviously, and Jaws 2's minor strengths don't make up for its minor weaknesses.
The plot is boneheadedly stupid: another shark pitches up off the Amity coast and starts eating the locals: Brody (Roy Scheider) is more and more convinced it's a shark as the evidence and the corpses pile up, and the mysteriously re-elected Mayor (Murray Hamilton) won't believe him because they don't want to jeopardise a huge property development scheme. Meanwhile a bunch of idiot kids (including the Brody boys) have gone out boating in the exact same stretch of water where it's been feeding....
It's great to have Scheider, Hamilton, Lorraine Gary and a couple of familiar islander faces returning. There's little worse than a sequel that doesn't bother with any of the established characters except in injokey cameos (or throwaway dialogue to explain why they're not in the new film) and simply remakes the original movie on the next beach south. It's also great to have John Williams back composing the music, which I've always maintained is a better, bigger and more enjoyable score than Jaws. The familiar Jaws motif is still there, but there's a greater exuberance in the music for the open water sequences, which all take place in dazzling daylight rather than the murky darkness of night.
And the shark effects themselves look fine: to be honest they're probably better than in the original (the one that eats Robert Shaw at the end always looked particularly ropey). But, for a movie that's mostly set in the bottomless ocean, there's no depth to anything or anyone. It's like they've stripped out all the character quirks and personality: there's no backstory or individuality to anyone (the kids are all teenagers anyway and they're a fairly bland group of largely indistinguishable hunks, nerds and love interests). So what's left actually has some of the feel of Jaws - the cast, the soundtrack - and some of the feel of a Jaws ripoff like Tentacles - the romping silliness, the lack of any depth. It's an odd mix, and it doesn't entirely work, though there are undeniably some pleasures to be had.