NO Shadetree - it is NOT currently possible to get the trailer under the boat until it actually floats- don't you think I would have done that? Also, dragging the boat further onto the bank will only make matters worse for obvious reasons
Bard - the overall beam (width) of the boat is 2500mm (Maximum legal trailer width). The boat is 5.5 metres long (20') so the dimension you asked about is about 21-22' At that point, the dam is about 4-5' deep with a steep downward slope
The idea of a coffer dam is a very good one but not practical due to lack of appropriate machinery and rock-hard ground. It would necessary to cut sideways (across the two bows of the boat) before back-blading out the spoil. I would not be game to get my Ford 6000 that close to the edge so the only way to do it would be to get in a dozer but that would probably create more trouble than it would solve (I'm no stranger to having one working here).
Yes, getting enough water out will raise the access hatches to the waterline and if that can be achieved, it will not be necessary to pump in air because the change of angle will be sufficient to get ramps under each bow and then start winching forward. Draining will then occur naturally as stated previously. This is exactly what has to be done