As i understand it you should always store a battery full charged, and before use say the night before stick it on charge again to restore any loss that may have occurred.
Although using 'buggy packs' an 'ok' battery to use in any model, but if your after run time, have you considered swapping to a 6v LA? you do find that these will still give out the required boat speed but also extended run time (around the hour mark or better all day in some cases)
Depending on your boat size will depend entirely on the size of the 6v battery required but possibly look for 6v 2 or 4ah batteries, a fair size not much bigger then a buggy pack generally but would give you around the hour mark on run time pending on your boats electrical setup.
I appreciate that cost may be an issue if your keen to stick with 'what you have' but i assure you that 6v battery's 'can' be vastly cheaper then a buggy pack's (depending on the 'branding') and charger around £5 mark.
There weight can be more then a buggy pack depending on the AH size but generally there around the 500g mark for the size that would do you well.
The advantages of LA is not only the extended run time that can be obtained but they also have a very slow discharge rate.
The choice is yours hope this may help in some way.