Actually, you first need to get a stamped RMV-1 form from your insurance agent so you can register the trailer.
If the boat is used, you should really get 3 separate bills of sale: 1 for the boat, 1 for the engine, 1 for the trailer. That way you can just pay sales tax on the boat and the trailer. Outboard engines are not titled separately in MA.
Did you already pay the sales tax for the whole package? If you did, then you just show the Form ST-6 to the RMV.
You can pay your sales tax on line for the boat to the Department of Revenue here:
https://wfb.dor.state.ma.us/WebFile/...axDefault.aspx