The EE network is regarded as the best for coverage on the canals, but is quite expensive (for comparable package deals) compared to Three, the second best coverage on the canals.
Your "best" coverage depends on the signal where you keep/moor your boat - if you can't get a Three signal, you might get an EE signal, or vice versa.
I just use Three, and if I can't get a signal I keep moving until I can. Other people have both EE and Three SIMs and switch between them depending on where they are moored.
If you are intending to move around the network, I would recommend getting a Three SIM and consider getting an EE SIM if you can't get a signal where you want one. Other people will no doubt recommend doing it the other way round!