health-wise, the only thing i can think of, based on my experience, will or is chronic shoulder problems from excessive paddling. but that took 15+ years to catch up with me. i am sure there are other issues (like skin cancer, pterygiums, etc.), but i have yet to experience anything else.
performance-wise - Its hard to be explosive while doing marathon sessions. hey, i am just as guilty of staying out way too long, disregarding all responsibilities, especially when it firing....most of us do that. but as i look back at my time spent in the water, i wonder if it did more harm than good to my surfing and especially my ability to explode when it matters.
i remember specific instances of my surfing going to shit after 2-3 hours....after 4+ hours it gets pathetic....perhaps its my age catching up to me. but i'd probably bet my house that multiple sessions of 30-90 min / day (scrambling to catch as many waves as you can in this time frame) with rest, naps and clean re-feeds (paleo as the foundation + dairy + lots of fruits and oatmeal) in between would do more for your surfing than one session of 4-6 hours followed by a binge, nap and perhaps another 1 hour afternoon sesh. you could probably liken it to pavel's quote of '... training as often as possible while being as fresh as possible...' you are just not fresh after 2 hours in the water, much less 4+ hours.
i also still believe that some degree of outside training is beneficial to surfing, but definitely should be secondary. i agree that conditioning is not entirely necessary (especially if you are hustling to catch everything in sight), though that would depend on the individual. multiple/short stretching (both static and dynamic) and foam rolling sessions are a must. yoga, i think, takes up too much valuable time. i am very skeptical of the use of balance training other than for the elite. i would definitely snatch, C&J, and squat, but perhaps i am bias because these lifts bring me more joy now than spending time w/ some of the morons that fill the waters now. however, the oly lifts could also take up too much time (to do it right), so that verifies my bias. that would leave squats (i would prefer that over DLs). My guess is PTTP or EtKB would probably be very appropriate, though i have done neither to backup my surfing.
anyways, my 2 cents...