I would be surprised if the seagull is still there when you return to the beach tomorrow. To me, your mistake was not calling a local vet / wildlife center when you came across the animal initially. You seem to suggest that these groups would not be interested in helping, and I wonder if this is an assumption you are making, or if you have had a past experience with these groups which has led you to predict their behavior in this way. Of course, your assumption that these groups would not be interested in helping the animal may be based on cultural differences-- I am from Massachusetts, and here I know of several local groups (including pet veterinarians) which help injured birds who are spotted in the wild (this may be rarer outside of the United States, however).
Still, I think it would be worthwhile to give these groups a call, even if there was only a small chance they agreed to help you. After all, if you called for help, you would have a chance of receiving help, and you would not face any additional risks (unless you consider a phone call a risk). But if you did not call for help, you would not have a chance of receiving help, and you would still face zero additional significant risks.
Because neither situation causes you to face any significant risks, and because one of the situations may cause you to experience a benefit, it would logically make sense to go for the situation which may lead to that benefit every time.