OP might get lucky. There is no entitlement on any Silver routes anymore, but the check-in agent at SHD a few weeks ago noticed the Gold tag on my bag and did not charge for a checked bag, although I think nobody would argue he would not have been entitled to do so. The miles even posted to my account, with 500-mile minimum and Gold bonus, as "UA Express 134" when the flight was without a doubt Silver 134. I think some systems must be lagging in reflecting the switch away from United Express.

There's probably less chance that you'll get lucky and not be charged at EYW since the change at SHD is recent and I think EYW has been Silver for some time. I would, at any rate, advise OP to look into Silver's policy on checking sports (specifically scuba) equipment. It is likely that they have a separate luggage policy in that regard.