How would a polytunnel be submersible? A poly tunnel is a really flimsy structure. Just going down 10 meters double the pressure (1 bar), and being any shallower isn't going to protect you from the surface water churn. Not to talk about the effort to push thousands of cubic meters of air under water. Not very practical I don't think. :)

Polytunnels are flimsy in compression, but clear plastic can be very good in tension and can contain a bar or so of pressure fairly easily, so it may be feasible. It was only one of the solutions I thought of for coping with storms if you are growing out at sea. The other two are either being able to reel them back onto land, or just making them short and tough, so they can just roll over the waves. The buoyancy problem isn't that difficult though as you can have counter-buoyancy that rises when they sink.

Building a submarine out of a polytunnel just isn't going to work. There are so many better ways to spend money. There are literally many thousands of km of dessert coastline where we can build greenhouses and grow vegetables where nothing else can grow. See my other entries in this thread.