EDIT: After using @Nick's method I have completed the tunnel. While the effort was indeed worth it, I think unless sponges come to SSP...next time I'll build a bridge. Next thing is a station...but that's another question.
Here's a picture of the finished product (the sandy island in the middle is what I was stood on in the picture below). .
I am attempting my first underwater MineCart tunnel and am having difficulty with the optimal (and easiest) method for building it. I've read this thread and have tried the "build walkway, drop sand in" method. I've looked at using doors and signs but this will probably leave me with a large amount of doors that I won't use (and don't want to dispose of). I've also though about damming the entire section off but that's probably a little excessive.
A tunnel cross section would be:
So far, I have got this far (1/4 of the distance) and have realised I am going to have to sort the current out as well: