Auto switching split charging is the way forward to keep things easy and simple if you are having 2 batteries.
I run one battery which is best of both starting and leisure (Semi-Traction), a 50hp motor isn't too hard to start by hand should the battery go flat.
I got mine (Varta Semi-Traction) from MDC - Motorist Discount Centre, Wallisdown. They ordered it for me in a few days and were cheaper than the web:
Motorist Discount Centre
Wonderholme Parade Kinson Rd, Bournemouth 01202 531065
There is a thread somewhere about buying batteries through a wholesale place that Derek worked for. Delivery is to Nigle's work (Nuffield Motors) - not sure if they do Semi-Traction but worth an ask.
I also have a battery conditioner that I plug in every so often to ensure that is it in tip top condition - Ctek Marine M45 : http://www.ctekchargers.co.uk/ctek-m45.php
HTH,
Rob