Doppler once bubbled...
Rule two...throw away the cave line that probably came loaded on the reel and replace it with "wreck line" which is thicker... now I always forget this but I think it's #36 braided nylon line (in any event its break strain is about 150 kg compared to cave line's 100 kg)
Rule three... do not overload the reel with too much line regardless of its design... keep the line well below the lip of the spool otherwise, when it swells after getting wet, it will fall off the spool at the most inopportune moment.
Line comes in 4 sizes depending on the size of the reel and the application. #24 is the smallest and may work ok for cav diving where lenght is more important than strenght, but in or over a wreck you need something stronger that can be a braded a bit more before it breaks. #36 is the next largest size followed by #48 which works very well on a wreck reel. Really large reels may come with 1/8 inch line.
The trade off for increased strenght is the reduced amount of heavier line that can be carried on the reel.
A 4" primary reel will come with about 400' of #24, 300' of #36, 250' of #48, or 110' of 1/8" line.
A 3" reel will come with about 175' of #24, 130' of #36, and 100' of #48.
You can also find 5" primary reels and they will come with 800' of #24, 550' of #36, 425' of #48, or 250' of 1/8" line.
In most cases it is advisable to strip off about 25-50' (depending on the size of the reel) to make it less likely to jump the reel. The line is rolled to the reel in a very orderly fashion at the factory, but does not look that way after the first dive and if you load to much on one side of the reel while bringing in the line, it can go over the edge. Removing some of the line helps prevent this.
Most traditional reel designs allow removal of the reel under water which helps clear a jam if the line ends up between the spool and the frame of the reel. (not losing the little plastic washers when removing the reel underwater with wet or dry gloves is another matter entirely and keeping a few spares in the dive kit is a good idea.)
I have pretty much been a die hard reef scuba fan in terms of reels but recently started using a plastic reel from trident to tow a dive flag. It is a lot stronger than I thought it would be and relatively easy to manage. It does have a spring release arrangement but it is very simple and much less likely to fail or bind the reel if it fails than some of the other release designs out there. On the down side, you need tools to remove the reel so getting the line jammed between the spool and the frame can be very hard to imposible to fix in the water if you don't catch it on the first turn or two.