Are these freshly wild caught or already established in captivity?
In my experience these guys are quite different from the more commonly kept water snakes (Natricinae). Here's a few things I've noticed and a couple tips;
1. Start breeding tilapia. You're going to need a steady supply of feeder fish that are big enough. I've only ever gotten one to eat rodents and it lasted for 3 weeks then never touched them again. In the wild, they eat primarily fish, amphibians, and maybe crustaceans (
1,
2,
3,
4). They will take frozen/thawed whole fish
eventually so you can euthanize and freeze the tilapia (or whatever) at the right size and not worry about keeping hundreds of tilapia alive. They will require live to start though. If you're worried about nutrition, you can feed the tilapia a high quality diet, cut the frozen fish open and put a small mouse in (doesn't always work), or use some sort of commercial carnivore pellet/gel and insert that into the dead fish.
2. Give them a decent size tank. atleast 120cm/4ft, preferably much more. They need room to swim, and also need a big enough basking spot. Stocks tanks work fairly well, and are relatively cheap given their size.
3. They can be fairly messy. Commercial filters work but may need regular maintenance depending on setup. I prefer DIY filters made from a pond pump and 5 gallon bucket personally.
4. They are rear-fanged, though I've found no indication they are dangerous to humans (of course somebody could be allergic). So you know, don't be stupid with them just in case.
4. Last but
definitely not least, get a fecal sample screened for parasites. There will always be a small parasite load in any animal, but there are some that "should" be removed.