If Prednisone is taken long term and at higher doses (usually over 30mg) then your body will more than likely retain water, and that's all it is. Prednisone can make some people much more hungry so if you do snack just try to stay away from salty stuff since that will only add on more water weight (and less healthy snacks/food in general can lead to general weight gain). Its not a fun drug but acts quickly (within a few days) so it depends on how bad things are.
Entocort has less side effects but its not impossible to experience some water retention (its far less noticeable and you'd have to be on it for a while as well). The problem with Entocort is that it takes longer to start working (can take up to a month for some people to notice significant change). So if things are really bad, Entocrot might not be the best option as Prednisone is fast acting and you may need that time to avoid further complications. If Entocort doesn't work then you'll likely be put on Prednisone which could mean more time spent on steroids.
What dose did they want you to take and for how long (did they suggest a short course of 10 days or so or did they slap on 50mg and say, "we'll see how it goes")?
You can always suggest Entocrot to your GI yet I'm sure there's a reason why they're going with Prednisone instead (could be because its faster which you may need quick healing or could also be an insurance issue, they may know your insurance wont cover it, Prednisone is much cheaper even with the generic version of Entocort so insurances tend to lean towards what's cheaper).