Read for term length, notice period to vacate, rate-increase language, and late-fee structure. Month-to-month with a clear notice period is the friendliest arrangement for seasonal owners; long fixed terms with automatic renewal are the ones that trap people.
Understand what the agreement covers. In nearly every case a storage agreement covers the space, not the contents — the facility is not insuring your boat. Keep your marine or RV policy active through the layup and ask your carrier about a reduced lay-up rate.
Finally, ask how they handle deposits and reservations, and whether the quoted rate holds for the season. A facility that will put the specific bay, rate and access terms in writing before you pay is the one to rent from.