There are several types of velcro, even one for maritime use. Try to boil the one you bought and see if it still locks after boiling and when wet.

Sewing a bag is relatively easy, but the mouth might be a challenge. You might want to add some cloth there to open it up.
1.5" velcro will create some friction when dousing and hoisting. Take care in the beginning, as the edges of the tape might cut your spi.
Also take care to make sure your bag will not slide on the hoop.
Stitching velcro can be a handful on a domestic machine, go slow and with short stitches.

Good luck with your project, and consider building your mould early as the retrieaval line might cut the glass on your hoop.