I've never used a title company or a lawyer for any boat purchase or sale. This is like a bride choosing to hire a service company to go through a name change. You can do it yourself, however, there is some due diligence required.
1) If there is a title, request that you be allowed to inspect it for any lien holders
2) If it's registered, request that you be allowed to inspect the registration for any lien holders (some no-title states list the lien holders on the registration).
3) Consider contacting the state about listed lienholders (this will not protect you from liens in other states, however, it is unlikely the vessel title or registration was moved with previous outstanding liens).
4) Talk to the marina where the boat is stored about unpaid bills or liens.
5) Include verbiage regarding the seller being responsible for any and all outstanding debts and liens.
If it failsafe? Absolutely not. I've been lucky.