Thought id give my take on this
You could use a PDA it would solve the touchscreen, space and waterproof issue (using a waterproof bag)
You could also get a MII 6000E which is a 600mhz Fanless mini-itx which i use and submerge it in a air tight container of oil or as tight as u can without the oil spilling.
A guy cools his pc totally in oil without any fans and thats a full motherboard
so a mini-itx thats ment to run without fans would be cooled enough
I use D3 frodoplayer and movies running at the same time on my MII 6000E with no problems at all so it's robust enough for the dooputer.
And you can also get a old PDA and use that as your monitor
If you use sidewindow installed on the PC it turns the PDA into a windows display so you can have frodoplayer or whatever running on the PDA screen and u can also use the PDA's touchscreen as the mouse with this software.
Thats my take on how you could make a dooputer
