hi,
a bloke at work had a similar problem. he used usb web cams for a security system. not underwater, but still had the problem of usb <5m. he got some cheep generic eBay USB to Ethernet converters. they worked a treat. but then you would have the problem of having more wires. they cant be run through a switch.
it has to be cam --- usb-eth --------cable------eth-usb ---- computer.
you would have to work out if any of the wires are not being used in the ethrnet cable to pipe you serial down.
otherwise, the suggestion of a Ethernet camera would work, though they are more expensive and heavier.