Jet skis (and the jet ski dungeon!) are great fun, but the skis themselves and parts customization are a bit disappointing.
Why are skis disappointing? There's not enough practical reason to use more than two or at most three of them (and I'm not counting junk skis), and bigger is always better unless going fast (which leaves anything middling in an awkward spot).
As for parts, all of the unique rare parts parts are scattered around the jet ski dungeon, and the non-unique good rares come from Ray's shop (with one or two also appearing on The Rig in addition to Ray's). You will occasionally find a rare medium engine or maybe a rare small battery as loot. That's it, that's the whole jet ski parts itemization in a nutshell.
For a specific example of the wonkiness of parts customization, the lightest and fastest ski can carry more weight than all other light skis and most medium-sized skis, as well as some of the large skis (!), and can move the fastest while doing so—because it can fit two small rare engines, including the unique one. The one downside is it can only fit a small battery, but that's really no problem as you can carry a pile of the very lightweight 200-capacity batteries with you everywhere.
So basically, the best skis are the Torpedo, the Protectorate Cruiser, and the Devastator, because the onboard weapons are all absolute garbage. The Shark and the Phaser are usable, but fairly suboptimal—especially the Phaser, since it's almost as frail as the Torpedo, but slower and lit up light a Christmas tree so that you're extremely easy for ranged enemies to hit. The light can assist you as well, but you can just wear goggles all the time like everyone does anyway.
All in all, though, I really enjoy jet skis. I just feel that the itemization of parts and differentiation of premium skis fell short of the mark.