After 90 days (I think) Maryland will want sales tax if you haven't paid it elsewhere. When we bought our boat we registered it and paid sales tax in PA. We kept it part of the year in MD and part in PA. When we started keeping it more of the year in MD, we switched the registration. We had to show that we had already paid our one time 6% in PA. The DNR is strapped for cash right now so they are not making as many runs around the marinas as they used to but when they do they are taking down registration numbers on the out-of-state boats, paying particular attention to Delaware.