I've been thinking about drink holders for my Skerry. The simplest method I can think of is a section of plastic pipe about 1" deep screwed to the centre frame above the seat adjacent to the sheer.  The drink bottle would sit on the seat and be held in place by the bit of pipe. I'd probably mount one or two on the forward and rear seat positions too for when I'm rowing from the front with a passenger in the stern. 

Doesn't have to be plastic pipe of course, you could make a retaining ring out of anything . . .

These are soft and can mount on a side of a NE Dory bulkhead.  These are the least likely to snag a line or hurt if you fall on it, no moving parts, nothing to break or bend.  Used them on multiple boats and they dont even yellow

http://www.westmarine.com/buy/beckson-marine--safe-can-holder--P015_454_003_508