You question does not make clear whether the dock slip is part of the real property, an easement or personal property, however, in California, if something is NOT included in the written purchase agreement, even if it is mentioned in the MLS, it is not part of the transaction. Our residential purchase agreement makes this clear. If it is not clear in your purchase agreement, it would be a good idea, in order to avoid misunderstanding and a dispute in the future, to either include or exclude the dock slip in writing.
Did you expect to receive the dock slip (or access to it) as part of your purchase? If so, you should clearly state such in your offer. The seller has a right to make any counter offer he/she wishes. You can counter or accept any or all of the sellers terms.