The rubber base/isolators required that a larger hole be drilled, and they were likely installed by simply expanding the rubber in the hole - not with a backing washer and bolt. Here's what I've done to boats (for the last 20+ years) that had such an installation:
the isolators have a nut embedded in the shank. You're going to cut that off - we're converting to a thru-bolt installation. Put the isolator in the hole on the boat. With under-deck access, use a razor knife to mark the shank of the isolator at the deck thickness underneath. Remove them and make a nice perpendicular cut to shorten the isolator so it does not protrude under the deck. Next, install good stainless bolts of the proper diameter/length, using stainless fender washers and locknuts underneath the deck.
The result is you have full isolation, yet the solid rigidity of a thru-bolt solution. Snug them down, but don't torque them too heavily or you'll destroy the isolator pad. Tighten until you see each start to bulge a bit. Another thing: to have the mount less likely to be twisted and bind, tighten the bolts evenly and sequentially in a X pattern as you snug them down - not one at a time all the way.