Since you have a 70-200 you should be able to answer both speed and reach challenges. I find the 70-200 has plenty of "reach" for water polo, especially if I can change my position near the pool if required. Yet some of the
pools are not well lighted so the f2.8 version is particularly helpful. If you don't have that consideration, good for you. In terms of surfing - don't often experience surfing at night so lens speed isn't an issue. All of Canon lenses work well in daylight, but some are better than others in weather sealing and salt air. I haven't had a chance with the new 100-400, but the old one works quite well from the shore. If the new lens really has noticeably better IS, then
it may be a better choice for shooting surfing and sailing from a small boat. you may also want to consider the
70-300L with a new 7DII as an excellent combination for your preferred sports.
I think that photo predates the 70-200 and the 100-400. I'll bet it was shot on sheet film too.