Tom
Harbor Models offer two alternative hardware sets; both having belt-driven MFA 800 motors. The ACTion P94 will be fine with either of these but NOT with the 850 motors which, as Stavros says, are prone to draw much higher currents than claimed - although with belt-drives the current will be less than with direct drive.
The brushless speed controller you have linked to doesn't have reverse - not many do. There are marine ESCs with reverse; these guys do a couple and there are loads more on the Internet if you include the word "marine" in your search criteria.
http://www.componentshop.co.uk/brushless/speed-controllers-esc/boats If the technical blub doesn't specifically mention the word "reverse" then you can assume that it doesn't have it. A number of folk use car speed controllers and program out the brake function. These are generally fan-cooled. As regards water-cooling, tugs will use large slow-revving motors so are unlikely to need any cooling.
SLA batteries are not capable of supplying large currents unless they're the heavy-duty and expensive "leisure" type. LiPos are popular because they're what the aeromodellers use and can supply very large currents. NiMH packs are a happy half-way alternative for those who believe all of the scary nonsense put around about LiPo batteries. Treat them properly and they're perfectly safe; act carelessly with them and they can bite - but so can a Stanley knife...............
Just seen Martin's post - the P40E is a better unit because it has four different modes of operation whereas P82 has only one.
Dave M