Could I please add that lead acid batteries MUST be stored fully charged or they suphate and lose capacity, but other types of battery including old NiCd, NiMh and Lithium types MUST be stored part charged - or they lose capacity - been there.....
Unfortunately I have no experience with sealed lead acid as supplied for model boats (but I teach sealed lead acid for motor vehicles!), so don't charge faster than the 1C (1 hour) rate of 4,5A for a 4,5Ah battery (and I wouldn't plan to discharge faster than that either). The 10hr charge rate generally preferred for lead acid would be 0,45A (0,5 amp) for a 4,5Ah battery - yes, I agree, keep it on charge for 12 hours at the 10 hour rate. Just recalculate for different charge rates or battery capacities.
A lead acid cell should read 2,1V when allowed to settle after a full charge, so the 6V battery should be showing 6,3V after a full charge. 5,8V is very close to fully discharged. I wonder if the charger is working, or if there is a break in or high resistance in one of the cables? or if the battery is simply past it's prime?