Sand off the bottom paint and barrier coat completely, but not necessary to sand off gelcoat. Use a small rounded tip carbide bur on a die grinder to lightly (and I mean gently) grind out major blisters, especially any that have any 'puss' in them. You want to do this now to maximize your drying time over the winter. It will be cold and freezing so the drying time won't be very efficient until it warms up, but better to do it now than wait until the spring.
If you want it done professionally it will be costly, but they will probably use a peeling machine which takes off the gelcoat as well. Its a viscous little beast that is kind of like a planner, but it takes much less labor hours than sanding. When drying time is complete in the spring, after filling the ground out blister divots, the bare glass can be rolled with straight epoxy resin or even a layer or two of 6 oz cloth can be added, then sanded and barrier coated.... you'll never have blisters again. Note, this requires removal of all your thru-hulls, but if you have blisters then then hull is probably old enough to have all this done anyway.
Is it stored indoors? If so, your drying time will much more effective.
