Most DIY and less than knowledgable mechanics use the el cheapo plier style crimp tool that comes in the "kit". It is almost impossible to get sufficient force on those handels to get the pressure that the whole crimp-on technology is based on. In order to get those crimps to really work, one needs a controlled cycle crimper that features a guaranteed crimp pressure before release. These crimpers always feature a compound action like a bolt cutter.

Neither crimp-on connectors nor soldering (or any other work on a boat that impacts safety) should be done by anyone "working above their pay grade".