The Nikon P50 would be great, it starts at 28mm, has manual controls and is pretty small, but there's still no housing for it.
I think that the P5100 would be a very good option. It's about the same size as the canon A7x0/A5x0 series (quite a bit smaller than the G7/9). The lens is only 3.5x, you don't run into the lens port problems like in the Canon G's. I have a P5000 (10Mp predecessor to the P5100) with no housing yet - but I recently discovered
THIS one in addition to the Ike and fantasea options. I bought The P5000 becasue I liked what I saw in the IQ and once I held one in a store it just felt so right. Coming from a Coolpix P4 the menu system was a piece of cake.
There's a guy on wetpixel that uses the P5000 in the Ike housing with Inon 67mm add on lenses (sacked macros and WA) and strobes (Z240 & D180).
HERE is a link to his Flickr if you want to see what the setup is capable of. P5100 will be similar as the additions don't seem to be very major.
Of the three housings, the most compact will be the Fisheye or the Fantasea. The Fisheye has connectors for mounting Fiber optic cables to it, it can be had for $150-$200 less if you don't want the Nikonos bulkhead, and it has a 67mm theaded lens port (but you need the AD for your lenses). The Fantasea costs about 1/4 the price and might be a little smaller. This one has a 46mm thread on the lens port, but Ikelite makes a AD lens mount adapter for their housings (
LINK) that fits on around the lens port. Richard at Reef measured the ID of the adapter for me at 55.8mm. The OD of the Fantasea lens port for my P4 is 54.2mm (at the glass) - 55.5mm (near the rest of the housing). I don't have the measurement for the P5000 housing but it looks to be the same so I'm pretty sure that it could work (might require a minor mod to get it to stay in place).