In Maryland you pay the sales tax when you register the boat with the Department of Natural Resources (DNR). The following is copied directly off the Maryland DNR website:
Your vessel, whether commercial or recreational, must be registered in Maryland if it is:
So as long as you can prove you use the boat somewhere else more than Maryland during the calendar year you do not have to register the boat in Maryland and thus, you won't have to pay the tax in Maryland.
Link here for more information: https://dnr.maryland.gov/boating/Pages/registration.aspx
SM #440 Cara
Deale, MD USA