Sun roof.
Seriously, I remember twitching the Zitting Fan-tailed Cisticola Warbler at Sandwich Bay, and being very impressed with a hide there made from a shipping container. Looked pretty vandal-proof, so doubtless long since burnt out. Heated seats would be nice, if done properly then they should just warm backsides, not the air
For sea-watching, in the best possible conditions a conventional hide is not much help, because the rain should be driving straight in. Some sort of large open fronted barn like structure where you could sit well back from the weather might be better.