The screws in your link are zinc, and since brackish is a mix,
it'll speed the rusting quickly. I used some zinc coated hardware
on my yak and once I took it in the York River (brackish), within
a few days rust started to appear - even with fresh rinsing.
Edit- They rust even in fresh water, but more slowly...best to use
stainless if possible. Or aluminum rivets.
Anyhow, DIY is fun, sometimes frustrating. Using silicone is the
right move, but the anchors you're looking at could present a
problem unless coated really well...