The deck screws will last 20+ years outside, but not below water. I’ve had them rust out in a years time on poorly draining decks, so you would t even get the year out of them if fully submerged. The stainless screws are definitely expensive, but will outlast the life of w/e your screwing into. Predrill the holes so you don’t overheat the screw and snap the heads because 1- they are expensive, and 2 you will burn up a sawzall bit very quickly trying to cut them out!

There is another option though that’s less expensive and that’s galvanized anchors. Just check not all waters allow for these (salt can compromise them overtime), but they will last a lot longer than the standard deck screws and are a lot cheaper than stainless. All framing nailgun manufacturers sell galvanized nails for all exterior applications. Can pop a few nails in and then only have to use 1-2 stainless screws to keep things secure forever. Nails will hold though because the wood will expand when wet and squeeze the nails tighter. Just if there’s a lot of movement and only nails are used they will loosen over time. That option will at least save you assembly time and also $ on not needing as many of the stainless screws.

Just my two cents hope it helps! I’m new to ponds but have been a builder for 20+ years. If you’re near Raleigh I’d be happy to help you out!