Here is their recommendation direct from the Torqeedo site:
Lithium battery life expectancy: The life of a lithium battery is barely affected by charging cycles. It has no memory effect. Therefore, the battery can be fully recharged after every trip, regardless of the charge status shown on the display. Generally, a loss of capacity of 4% per year can be expected. Ageing is accelerated if the battery is exposed to high temperatures for long periods while fully charged.
So, maybe stick the motor in fresh water and run it down a bit, but heat seems to be the issue there.