In MA, you can deduct the portion of gas that is Fed Excise tax (regardless of where it was purchased). It was 21 cents per gallon this past year. I've been told that it is going away, but it is still here. Here's the details from BoatUS site along with the info to get the forms from Mass department of state revenue.
Massachusetts: Recreational boaters can apply for a $0.21 per-gallon refund (before factoring in a 5% user fee on the total refund amount) of state sales tax when they purchase highway fuel for use in their boats. To claim a refund on the gas tax, one must file within six months of the purchase date, and include all original sales invoices with the application. For a diesel fuel refund, one must file the claim within two years of the purchase date and include all original sales invoices with the application.
The gas tax refund form, GT-9M and diesel fuel refund form, SFT-9, can be downloaded from the Internet by going to
www.magnet.state.ma.us/. First, click on Taxes/Revenue, then click on Tax Forms and choose Miscellaneous Tax Forms. For more information, call (617)887-5070.