Trade them in for tortoises, which are much better suited to canal boat life. Apart from the far closer match of speed, they're much more resistant to the effects of sustrans tow path 'improvements' and I've yet to see a post about them falling from gunnels (do not confuse those escapee terrapins with lost tortoises). In any case, they look like they could be modified to float quite easily. They're also remarkably eco friendly for an alien species, as they're unlikely to escape and turn ferral and will not fill your engine 'ole with recently deceased native wildlife. A Dandelion leaf diet is cheap and easily sustainable. When they finally depart this mortal coil, they'll already be a family heirloom and you'll probably have gone years before them, so they pose none of the usual upset associated with pets (well, maybe for your grandchildren's grandchildren). In winter, pop them in a cardboard box to keep the frost off and break them out next year for a summer of tortoise filled fun and high jinx!