can you use one for cranking?

You can but it isn't the best situation.  Lots of guys who run 36V systems and don't have room for a 4 th battery crank off a deep cycle.  

can you use one for cranking?

I do and have for a while. My battery powers my trolling motor and starts my 40HP outboard. The starter draws less current when stating the outboard than my trolling motor on high. Plus it only runs for 5 seconds tops each time I start my outboard.

I guess it all depend on how big your outboard is and whether or not it is fuel injected or not.

I have used a deep cycle battery to start my 225 Yamaha for several years. This battery powers two fishfinders, GPS, several pumps for the livewell and running lights(night tournaments). Just to much for a cranking battery........................................Al

Yes it will work, but the deal is this. The starter might draw the same amps as a trolling motor, but its drawing them in a very short time. It is hard on a deep cycle battery, but it will work. Solve all your issues and buy an AGM battery.

I think your best bet is to get a dual purpose battery.  It has more cracking power then a deep cycle, but it also can be drained down and charged more often.