So the questions probably are:
Can a decent drone that is up to this kind of thing be had for not too much money (I can see drones from £50 on Argos) (if you want usable pictures then no)
Can a drone that doesn't weigh more than 250g be found that can do this? (250g is a joke limit, you will need to go over that for anything worth having, 250g is literally toys in tesco)
Endurance - what's the solution? Spare batteries? Fly back, swap out battery, fly out again? (spare batts yes are the solution)
Stills - do you just take a movie and select a still? (depends on the camera, a decent one (i.e not a 50 quid argos special) you could take a still from the video but still option will be better as it uses all of the sensor so the resolution goes up.
you will need to land it on the boat, not impossible to do with some practice.
i would look for a DJI phantom 2, if you need to see what you are shooting then a phantom 3/typhoon q500 etc, i don't see the point in point and hope photography but it is a lot cheaper!
if you want something basic that you can put an action cam in- BAYANGTOYS X16 Brushless Altitude Hold 4CH 6-Axis Gyro RC Quadcopter RTF 2.4GHz
if you want to up the money then you can start getting brushless gimbals/better cameras/fpv etc.
keep in mind the cheaper fpv ones are wifi only transmission so you are limited to maybe 400m or so, plenty for your usage.
i stress though, you only get what you pay for with this stuff, there is no real "bargain" drones out there, they are cheap for a reason but if they go for a swim they don't hurt as much.