given your current battery size, 12v 7ah ,.. there quite heavy 2.4kg last time i checked, the likely hood of getting longer run time with the current battery is slim, as for lithium's, not quite sure there up to the job.
The way to incress performance is to change to larger motors, 800-850 or bigger sizes will incress performance but your still looking at running with LA, you could just look at a higher AH of LA , say a 12v 12ah or possibly a small 24v battery will do the job.
Be honest with yourself though, performance comes at a cost of £30-50 per 800-850 size motor or upwards of £100 if you get the real large beasties. adding more weight in a larger battery will decrees performance as extra weight will be added.
Is it really worth spending x amount of cash for little change in performance?
Or is it worth just changing the motors to larger sizes, and keep the battery as is, and carry spares with you for the lake days out?, you obviously transported the boat in a car, im sure there's room in there for a few extra battery's.