Fix dropping half of the coal into the ocean in order to get stuck halfway doesn't make sense. First of all, it would put everyone's life in danger, including his own. Second, Fix wanted to arrest Fogg on suspicion of being the London bank robber. In order to do that, he has to wait for them to reach British territory, so there's no reason to stop Fogg from reaching the UK. Fix somehow seems more keen on making Fogg lose his bet, for which he has no reason at all.