In my observation, plumbers usually use a high quality clear Silicone. It is the choice that has been around for decades, worked for decades, is stain an mildew resistant, and doesn't get them in trouble with the customer if there is some residue (because it is clear). In my observation few have had any reason to change. I've occasionally seen them use white if the sink is white.
I don't like Silicone anywhere near my boat, so I'd probably use some other "kitchen and bath" caulk that lists itself as paintable.
If the sink fits well, I'm nitnsure there is a reason to weight it. I'd only do that if there was more of a gap than I wanted, it didn't take more than a little force to hold it, and I couldn't fix the root cause. Or, I guess if I expected wild boat moment at a rough mooring or anchorage or something.
I wouldn't spend the money on topside or bottom side caulk for that purpose. I just see it making it harder to get it back out if doingnsovis ever needed to get a better angle on a valve or other plumbing for service.
