I hav been using SLA (Sealed Lead Acid) batteries since 1972 both professionally & as hobby batteries. I concur with all of the earlier remarks. In my experience the situations of outgassing, leakage, & battery failure hav resulted from OVERCHARGING, either thru excessive voltage being applied from the charger, or excessive charge current. Simple chargers often lead to problems as they are unregulated, simple devices, designed to meet an inexpensive price point.
A safe charger for a 6V SLA supplies a max of 7.4V, voltage regulated, max current 1.35A to a 4.5Ahr battery....so a minimum recharge time is approx 4 to 5 hr. For a 12V SLA the max wud be 14.8V (this # suggests, BUT do not use a charger intended for a standard lead-acid car battery)
SLA has several advantages for the model sailor: relatively inexpensive, very available in a variety of physical sizes, standardized terminals, can safely be charged without removal from the model, serve as ballast, & can be safely connected in series or parallel without using isolating diodes.
Yes, I do use LiPO's in certain vessels,for lighter weight,longer runtime,but remove them to protective bags for charging & storage,then reinstall immediately befor a run. In comparison I hav found SLA to be relatively carefree.
My years of experience with NIMH in model aircraft have also taught me that they are an inexpensive, lightweight, easily-managed option.