guarding the waterbowl is unusual , might be a pretty possessive dog ..........what do you do when he does this ? what do the small dogs do ? with the size difference you may need to back up the small dog , my dogs are all big , i'd let them take care of it , he's most likely bluffing , but with the size difference , finding out might not be good ........ you may need to take the bowl from him an give it to the small dog in front of him , if he tries to drink with them let him know he can , if he tries to take it from them , don't let him , take it away again and give it back to the small dogs , and take the small dogs side ......... you may need to do it with something like chicken broth , water may not be enough to get the behavior your trying to correct when your looking to correct it .... there are plenty of folks here who will tell you to "not set the dog up for failure" , i'm of the opinion with behaviors like your describing i'm a believer in you need to get the behavior so you can correct it .......... it's not something you can ignore , if you can get them drinking together and doing it as often as possible without issue , the longer it goes on , the better your chances at stopping the behavior ..... I just don't think you can stop it without correcting it ... you may also want to try putting the water in a new type of unfamiliar container and starting over under supervision , backing up the small dogs ............