There were moments in alignment support where I lost a ton of hair and brain cells. The hardest part was getting W3C compliant rendering of SVGPath on <canvas>, after that it was all downhill.
I wrote the beginnings of txtjs on a trip to Bonaire in June. I find I am about 2x-3x as productive writing software during surface intervals while diving than any other time. The diving forces you to relax, think, breath, and you cannot use a computer. When the dive is over, you are 3x productive and things are easier to implement. Also I dive with Nitrox with adds 10% more O2 into the tank, I end up sleeping better, less risk of n2 narcosis, longer bottom times, and I can think very clearly after a dive.
It sounds absurd but I have found I will hit flow more on dive trips than any other time.
I am, although NAUI. Diving leaves me surprisingly sleepy and unmotivated (I don't do any blended air stuff). The idea of cranking out a typesetting engine on a dive boat was a new one. :)