If the seacocks are of the ball valve type, you can try slipping a piece of 1" PVC conduit or pipe over the handle to get a bit more leverage and also more comfortable grip. If that doesn't work, it might be best to replace them. The balls in some seacocks may, when not regularly exercised, corrode so much as to become very hard to turn. Since the ball valve seats are made of plastic (ptfe or nylon or some such), penetrating oil
could make things worse. I think silicone grease or lanolin or vegetable oil
would be safe, although, for example, Apollo/Conbraco does not recommend any
sort of maintenance
for their ball valve seacocks. When you finally get them freed up, you should exercise them regularly (perhaps every week).