You could always scratch build a naval mount and put the spare one on a gunboat, Grimm:
Eureka do a very nice naval version, along with a Hotchkiss 3 pdr and some gun crew, for anyone looking at building gunboats.
The shipboard conversion is a very good idea, though, as I was wondering what to do with my spare.
http://eurekamin.com.au/product_info.php?cPath=87_126_139&products_id=9570 I can't seem to copy the large version of the pic

You'll have to look at the website if interested. Available in the UK ( from "Fighting 15's"? ) if shipping from Aus is too much. I have one of each gun along with crew and they are lovely models. If memory serves, they are reasonably compatible with Copplestone etc, but they are "somewhere safe" so I can't find them at present.