The best padlock for a boat locker is one that can withstand harsh marine environments. The Abus 24 series is a range of discus padlocks made in marine-grade stainless steel. They are effectively a closed shackle design which helps to prevent people from using a hack saw to quickly cut it off as they might a normal stainless steel lock. Abus are the only manufacturer with the technical ability to make a disc lock so small and also make it reliable enough for a marine environment.
Most sailing boats have lockers in the cockpit and are often subjected to rain, saltwater spray, and the general saline atmosphere in which they sit. Almost every sailor will have had to deal with a rusty padlock staining their deck or struggling to open it after it has been left for a while.
Often chandlers tend to sell weatherproof rather than marine-grade padlocks. The difference being that a weatherproof padlock is chrome treated whilst a marine grade padlock has stainless steel mechanism and shackle. A big difference.
The other problem is that when you do find a marine-grade padlock then it may not be secure enough since stainless steel is a soft metal. Marine-grade locks come with stainless steel shackles which makes them easier to cut. Also, most boat lockers have little hasps that take a maximum of a 7-8mm shackle thickness so you can’t just go and put a really hefty marine lock on. Normally the only way around it is to get a small closed shackle marine grade lock.
Unfortunately, you will have trouble finding a closed shackle marine grade padlock with a shackle less than 7mm in a UK chandler. The only closed shackle marine grade padlock on the market with a small enough shackle is the Abus 24ib/50, with its shackle diameter of 6.5mm. Available as key differ or keyed alike to suit your requirements.