Is it s good idea to use washers on countersunk self tappers

[TiggerToo]

I have a boat with a "toerail" fixed to the deck by stainless steel self tapping screws. Leaks have developed, probably between the screws and the aluminium toerail. Would it be a good idea to look for some sort of "washer" the put between the screws and the toerail (the whole system would benefit from electrolytic isolation anyhow)? Where can I find them? The material must resist compression and be watertight.