You definitely need the advice of a lawyer--and not necessarily an estate lawyer. The issues are NOT simple.
IMHO the basic and essentially first question that must be answered is what, if any, property interest did your fiancée have in the boat.
The second question then is what happens to that interest on his death.
The first question can involve questions as to personalty (personal property rather than real property/real estate for which the rules are very different) and/or admiralty law. It will also include the question as to what is the extent of your fiancée's interest (and therefore his estate's interest) in the boat bearing in mind that he made no financial contribution to its purchase
There is also the possibility that a court may determine that your fiancée's interest in the boat was held by him pursuant to a resulting trust
(google it) whereby whatever interest he (and thus his estate) had in the boat is determined to have been held in trust for you and thus do not form part of his estate (ie. all yours)
After your lawyer has identified the nature
of the interest of your deceased fiancée (and thus his estate) in the boat then the second question needs to be asked is what effect does his death have on that interest ---is it extinguished? is it a joint tenancy interest which passes by survivorship? does it form part of his estate?
The answer to the second question will likely depend upon the answer to the first question and will vary depending on whether your fiancée's interest was an interest in personalty, or an interest under admiralty law, or whatever and whether his interest was absolute and separate, a joint tenancy interest, a tenancy in common interest etc
If your fiancée has, in law, an interest in the boat which is not extinguished or does not pass by survivorship then that interest will pass to his estate and will likely be administered (dealt with) under the laws of intestacy (administration of an estate without a will) of the applicable jurisdiction which probably do not provide much leeway in dealing with it.
You need the advice of a lawyer --he will want to know all the facts surrounding the boat, its purchase
, its insurance
, any other payments made (and by whom) in connection with the boat (eg: moorage), its registration
, other property bought or held by you and your fiancée etc
Finally if you delay unduly in seeking the help of a lawyer then your claim may be prejudiced by such delay
So you need to GET A LAWYER!!!