I personally prefer two motors driving two props, you'd have to either invest in twin ESCs or a single ESC able to handle the current load of the two motors. If you chose to go with the single motor route, you will have to use a gearbox to drive both props, this eliminates the need for a second ESC altogether. As for the propeller seals, stuffing boxes are very popular, there are also specially made submarine seals for this purpose:

http://www.subtech.uk.com/acatalog/S..._Products.html