if he drank that much water he may just have less control on his bladder right now. i'd make sure to take him out more often and praise, clean the accidents with a proper odour neutralizing product and toss it up to a bad few days.
be alert, he may be giving you a cue that you are missing. my dogs are not used to "asking" to go out as they get out often, so sometimes their cue is vague (or they don't have one) so it's tougher in situations were they need to go out. This was an issue with Penny more, until her bladder became much stronger. But still if she has total freedom all day and needs to go she is likely to just find a room she doesn't use much and go (VS if i keep her on the main floor she is gonna hold it in).