Some ESC specs can be very optimisitic. This can be especially true of some of the cheaper ESC's from the Far East.
If the controller is well made and designed, then it's unlikely you need anymore than 10A in the vast majority of installations. To power a submarine of modest size at moderate speeds only needs a few watts usually. These days it can be quite difficult to find a controller in production with such a low continuous current rating unless it's very small. Most controllers start at 15A and go upwards. The controllers I use allow 25A continuous operation, and are small enough for most models.
Having a larger controller doesn't hurt, but no sense in paying over the odds for a controller that can deliver more than what you require.
For a boat of around two feet a shaft size of 1/8" or 3/16" is more than adequate. Here in the UK, we tend to use metric sizes,a nd 4mm is a popular size for shafts, with smaller models sometimes dropping to 3mm.
I recommend you get this book if you're serious about scratchbuilding a sub- http://shop.traplet.com/product.aspx?c=294
It's a tough read, especially if you lack a technical background, but most bases are covered in this book. It deals largely with theory and concept, there is little in the way of instruction on how to prqctically build the items beyond a few hints and tips. What it does allow you to do is calculate things like hull strength, displacement, sealing, power requirements and ballast systems. It is by no means exhaustive, and I'd avoid building any of the electronics in the book, as they are rather dated, with out of production components listed, plus some of the schematics are a bit suspect.