Hi I'm a big surfer (or have been most of my life but doing less now at 40!) also building a mobile game in Unity/C# right now although it's not surf related like yours it's actually based on Chinese Wuxia cinema from 5 years spent in China. I considered building a surf one and was looking at potentially trying to 'gamify' a slightly different element of the 'surfing' experience which was to focus on predicting/catching and then staying on a wave at a busy break...so top down early GTA style graphics initially waves would come almost breaking in random spots in relation to previoius waves/sets, but never identical, and you'd have to kind of 'guess' ahead of other AI controlled surfers where to place yourself then paddle for position then try to dodge others on the way in....I thought it could be kind of fun in an arcadey way and not require so much of the physics you're describing, which is no doubt really hard to code...I'd love to see your game though!

I guess the trickiest part is fluid dynamics. Are you using a physics/fluid sim or handrolling your own?

How's it compare to kelly slater pro surfing