Propane hot water heater location--rendering

Just behind where the unit is to be is where the shower area is located.
http://s1216.photobucket.com/albums/dd377/projecthouseboat/?action=view&current=ae965c29.jpg

Keeping the water lines short should help with temp adjustment and quicker heating. The wafting of fumes up and around into cabin (station wagon effect) could red X the idea for a surveyor, but could the flue directing spent fumes over & away help? Also, of course I will install CO and propane sniffer alarms. In addition, the burner will only be operating for a relatively brief time (enough for a quick shower) so despite the possible wafting in of fumes, couldn't that effect be acceptable in brief because of the relatively short burn time of the tankless hot water heater? Hope the renderings help:
http://s1216.photobucket.com/albums/dd377/projecthouseboat/?action=view&current=f557792d.jpg
http://s1216.photobucket.com/albums/dd377/projecthouseboat/?action=view&current=99d98a56.jpg
http://s1216.photobucket.com/albums/dd377/projecthouseboat/?action=view&current=d0ec816c.jpg
http://s1216.photobucket.com/albums/dd377/projecthouseboat/?action=view&current=951caadc.jpg

I still don't see how my on-board water pump would have a problem providing the required 2.2GPM [minimum water flow threshold to allow the burner to keep firing.] I expect the on board water pump to increase output when there is a demand in order to maintain that min. 2.2GPM threshold. I haven't started on the water system yet. With this 2.2GPM minimum threshold flow rate that must be maintained to keep the tankless (propane fired) hot water heater going can you thank of any other special considerations for the main water pump aside from it being large enough and an on-demand type?

I will be located in an area with maybe no access to power, but with access to water (one of the reasons for going this route with water heating.)
The boat is converted to outboard so no heating from exchangers.