I don't know about specific scientific stuff per se, but I'm decent and making science-sounding fictional stuff, which I think is more what you're looking for, unless you're playing with a physicist and a biologist in the party.
1. Reverse-electrolysis. Hydrogen and oxygen naturally combine to form water, the process simply takes too long for what you want, so super-speed it up. The bomb carries two huge tanks of compressed gas: one of hydrogen, on oxygen. When "detonated", the two gases are thrown together, applying the necessary energy to force the chemical change as quickly as possible. The only structural problem with this idea is that unless the bomb is absolutely gigantic, it won't be able to contain the water while still producing, and will either need to be vented or will rupture the structure of the bomb prematurely. The bomb would have to be capable of carrying a little less than the entire mass of water it will produce, such that once it reaches its maximum capacity and the last water is made, it explodes, releasing the torrent.
2. It sounds good. Like I said, you don't really need actual scientific fact, just good fictional explanation. I'd go with that idea, assuming these organisms also take in other chemicals/nutrients for those biological processes.
3. Is the creature sapient, and if so, intelligent enough to work a device such as a walker-bot? If this is true, strapping it down in front of controls (straps/controls that its already existent appendages can work) and let it do the work. If this is some mad-man's evil plot to make these creatures walk on land and wreak havoc, then a neural interface will need to be installed, connecting directly to the brain similarly to its existent nerves and muscles.