200 pound flat line will catch anything you might want to keep....400 would be scary...and expensive and hard to find...I use the flat line on sailboats all the time and really not much fun dragging fish to death but okay if you will eat it.
You might try looking in Home Depot or Lowes for a big spool of heavy weed eater line. I bought a 2000' spool of 2mm diameter line several years ago. Don't know the test, but looking at it I would guess it has to be at least 200lb.
FTU / Cut Rate has the stuff. We bought some for $16.00 or close to it. Maybe 100 yards or so. It was black. We put it on an downrigger before we heard all the stories of the mono splitting the spool. Now we need to get rid of it.
I posted earlier about using 200 pound mono instead...I also use a cut off rod handle and a old reel with the drag locked so you can reel it in when you are through...100 yrds of 400 pound mono is impossible except on the biggest of reels and then its a pain... a check cord and snubber also are handy...this alows me to fish any where in the world while on a blowboat...I also use the american rodsmith two piece helicopter rods...a couple of pics...first one the flat line in the lower corner on the stern rail and the second picure is self explanitory...
Just another way.... after pulling out one of my old cuban Yo-Yo's http://http://www.palmbaybaitandtackle.com/products/cubanyoyo.htm this week end for a trip down Island and seeing a sugestion... sometimes the simplest is the best...a couple hundred feet of 3/16" nylon ( easy on the hands ), a swivel, twenty feet of 80# monofilament, a swivel, 8 feet of wire leader. Hope this helps and sure is inexpensive