There is no lock that will stop an outboard being stolen from a RIB or SIB. If the lock is strong, they'll cut the transom or the tubes. However, a decent lock may stop the opportunist, or if the boat is left in a place where the use of a jigsaw or angle grinder would be conspicuous. I was very satisfied with one of
these:
However, I was disappointed to discover that it did not float.