Mab, I use stainless too. Usually 10 or 12mm bolts.
Whilst stainless is less conductive, it is also using a length of only 1/4 inch or so over a 100mm ( 6x6x3.14) cross section.
The losses are so small as to be not worth bothering with,... never felt a warm terminal.. so I can't say I'm about to change.
In a marine environment.... even less so, however, copper bolts and washers would be better I suspect electrically..... but low strength.
The slightest loosening of the terminal will have a huge effect, far more so than materials used,..... and lug performance too.
.............oztules