I have a Kayak much like pic related that I'm thinking of

>>1193200
psh. no worries. just make sure you get bolts with rubber washers/rubber coated metal washers, and torque them down well.

Easiest would be a stainless steel carriage bolt, add a rubber gasket/washer under the head, drive it in and torque it down with a spacer and a washer (so the nut doesn't dig itself into the inner side and get stuck).
As long as the bolt is tight, the gasket will compress and seal, and the carriage bolt SHOULD be smooth enough to not cause much drag. Lock washer under the nut.

Basically the thing to worry about is how to secure the bolt to keep it from spinning when you are trying to tighten down the nut, and not add to the drag on the outside. Use stainless, or be prepared to replace every few years because rust has opened up a channel for water to leak in.

>>1193200
drill is the best way to make holes man

Oh yeah no worries. Just take your time. Make sure you get you size your holes and hardware carfully.
I put some strap mounts on a couple a weeks ago so I would have a removeable deck for the doggo to stand.
I used a cheap hand riveter, and a little bit of hard resin.

I have cheap lifetime boats but I've always had good luck with a heat. Blowtorch to melt over breaks and holes. If you put a screw into the plastic you can touch over it for a bit with a soldering iron to melt the plastic into the threads and seal it. careful not to get things too hot.