You need 3 things, those being

1. Diving Fins
2. Scuba Diving Fins
3. An Underwater Scooter
4. A Battery

After researching them, you can make diving fins with 3 marlin scales (which you can get via killing marlin) and 2 rope which you get via cloth. For the scuba diving fins you need 10 rubber (Which is found by harvesting tires) and 6 duct tape which can be found in tool boxes and safes. And Underwater scooters can be gotten by buying it from a trader. I'd recommend using those items with the $ in the corner that are sitting in your storage doing nothing as a currency to buy it. For the battery you can: 1. Buy it from a trader or 2. Craft it with 15 scrap 2 copper bars and 3 electrical parts.

Now all you do is put the diving fins in the corner 2x2 area in your inventory, put the battery in the scooter and test out your speed! Also i would recommend making battery chargers for you and your friends (If you actually want to be kind to them after they threw a MILITARY GRENADE AT THE BASE)

Have fun Goofy Goobers!

(By the way you are now faster than all boats with everything combined and with the fins alone you're faster than all boats but the Jet Ski,) Edit for people who saw this earlier yes this remains true.

If you want to see my full guide to Sunkenland, click here
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3132465179