Hi Philshoz
I've been a surfer since about 1974
Taken many surf shots from land and water.
I guess it comes down to budget.
I'd love a big grey but can't afford one.
Best lens I had was a FD400 F4 I got back in about 1984
Now I use a 70-200 F2.8LIS I also have a 2xIII
I also do a fair bit of birding and am considering either the old 300F4 or the 400F5.6
Both would be good cheap lenses
The issue is surf spots can be close, or a long way out ... where you could almost get away with a 100mmm or need up to an 800mm
For water shots I have a 14mm ... in the barrel type stuff .. I also use a 35mm ... but also a popular combo is either something like the 24-70 and even the 70-200 is a popular choice, for bigger waves where it's hard to get in close, by sitting in the channel, or if you have the luxury of being on a jetski.
The cheaper lenses are a little slow for cloudy days, but these days cameras are good with high iso .. back in the day with film, my old 400F4 using 100asa film would be too slow on cloudy days.
So for me either the 300 F4 or the 400 F5.6 would be my next choice in lenses ... but I am hoping for new models shortly from Canon as they are the two oldest lenses in Canon's line up ...
The 300 F4 for me is high on the list as it has a short focus of 1.5m good for small birds .... the 400F5.6 only goes down to about 3.5m min focus.. way too long focus for small birding ... fine for surfing though.
So I would suggest the 400 F5.6 over the 100-400 .. it does have a better image quality.
There is talk of a new 100-400 and could be worth the wait, so that would be an option ... but I'd get the prime 400 over the current 100-400