The best crimper for spearfishing ie. for mono and wire is the Speardiver ALL stainless steel crimping tool available from FreediveStore.com. It has a stainless steel tool head with stainless steel handles and red non-slip grips, with 4 positions to crimp all wire and mono normally used for spearfishing, from 50 LB to 450 LB test. And includes a built-in side line cutter.
Being all stainless steel it's perfect to use at home and take on the boat. Other crimping tools have stainless Steel in the description but are almost never completely made of stainless steel. Normally only the handles are stainless steel, this is most evident with crimpers that have a black color tool head. But even crimpers that have a steel colored tool head are most commonly not stainless steel
For best holding strength and durability crimp in two spots slightly back from each end of the crimp, so that the crimp edge doesn't dig into the mono and cut into it. Leave the middle un-crimped. When possible lightly melt the end of the mono with a lighter to form a glob, before pulling it to the crimp. This will lessen the possibility of the mono slipping through the crimp.
Speardiver ALL stainless steel crimping tool