$2 Nail polish. Make sure you wink at the cashier when buying it.

 As an ex-crayfisherman, i've seen every aapproach, and tried most myself. First it was all oil based, either enamel or the hi-vis fluoro colours. Then we went to water-based, as the acrylic paints got a lot better. You can get a one litre plastic pot of fluoro green, yellow or red from anywhere that sells pro gear , for not much money, that will last you for a decade of painting four floats. ( Pheonix brand?)As an example, I still am using one of these pots that was left over when I finished with crayfishing in 2005. 

When you say "last", do you mean fade, or get dirty? They will all get dirty--the normal habit of just pick and drop in the whites sees ropes and floats get dirty real quick.  We always had to clean and paint floats after the whites, would dip them in a chlorine solution then scrub off what was left.

Oil based enamel any colour you can imagine from Bunnings

bought little tin of black for numbers and little tin of yellow.

get four large floats out of each tin of yellow.

still no issues after 3 years in water. Not faded or chipped just a little dirty

cheap as shit. It’s a small small tin about a third size of a coke can 

 As a painter and decorator, I have used Solver Duraguard Gloss Exterior Acrylic on Mine. Has lasted for the last 5 seasons and still good. Easy...