The image quality of 50-135 is certainly good enough if you can get that close but given that whales are quite big creatures, the boat you are on might not want to get too close in case the whale hit it..given that it is unsure how close you can get, I will suggest 50-135 + 150-450 combination for the best results..haha..but seriously, if you dun want to spend too much, I will suggest 55-300 (either WR or non WR)..light should be good so you will not need the f2.8..
personally, if I dun wan to spend so much on a new lens, I will get a non WR 55-300 and use a plastic disposable rain cover..the lens should be pretty cheap in 2nd hand market...and if you want, you can always buy those super big carabineer and hook your cam strap to the railing pole..just in case you drop your camera, at least the camera wun drop to the bottom of the sea but whether is it still working will be another matter