No fishing line is crap as it is too slippery to tie knots in easily(except for fishing line knots which are too big and messy if you get my drift).
Best thing is the thread that looks like slightly thicker sewing thread that is used for tying the guides onto fishing rods. Lots of guys build their own rods so the thread comes in lots of colours at about $5 for half a kilometre roll. Any fishing tackle store will have it. Use a standard thick needle and a thimble (trust me it's hard to push the needle thru the fabric). Tie knots off well and dab with a tiny tiny bit of 5 min Aralite. This stuff is good as it is very strong, synthetic so it won't rot (standard dressmakers cotton thread is, well, cotton..... so it lasts 5 min after being left in a wet sailbag.
The fishing rod binding thread works, I had 4 parallel rips about 30cm long in the luff tube of a sail (courtesy of Gnaraloo reef). Took me an hour to sew it up but left it like that for a season no worries. Never go on holidays without it.

I'v used nylon thread from fishing stores used for binding eyelets onto fishing rods .It's strong, ties well and best of all, it comes in pretty colours. Dental floss works well if you want to use your sail to clean your teeth with.

Dentist told me that you don't have to floss your teeth only tha ones that you want to keep.