I'd not recommend connecting a 30 amp (potential) load to a 15 amp outlet. While all that would happen is the shore power outlet CB would pop when you try to draw more than 15 amps you never know who wired the outlet. Given it is a tad odd I'm thinking they did not know marine wiring.........
The NEC requires that shore power for boats "be provided by single receptacles rated not less than 30 amperes." [2011 NEC 555.19(A)(4)] and that each such receptacle be supplied by an individual branch circuit (no more than one such receptacle on a single branch circuit).
If there is a 15A or 20A 125V receptacle at a dock or slip, it's intended to supply other than shore power, as in small tools, bubblers, whatever. In fact, if such a receptacle is provided in the same enclosure as a shore power receptacle, the 15A or 20A is required to be marked to indicate that it's not to be used to supply power to boats. [555.19(B)(2)]
At least, that's how the marina gets away with a small receptacle at a slip. It's not 'intended' to provide shore power. A technicality perhaps, but a decision by the marina owner or whomever nonetheless.