Well said, Monsieur!
Far too many modellers are fooled by those seductive capacity ratings, to the extent that some believe they can even run two brushless motors from one SLA battery and obtain an hour's sailing from one charge. If I've seen one Huntsman or Perkasa lumbering round a pond like a narrowboat, weighed down by a huge brick of a battery, then I've seen a hundred. It makes me weep. It takes a great percentage of the motor's power just to get the battery moving, never mind getting the model up on the plane. Fitting a bigger/faster motor will simply make it worse in that it will drain the battery even faster. SLA batteries are large and cumbersome things and in consequence can be very awkward to fit. They have to be charged at a relatively slow rate and so can't be recharged at the lakeside (unless it's overnight, maybe). OK - if your tug operates fine with them then carry on, but you might get a helluva surprise if you replaced them with modern fast-charge high-current cells.
Unless they are of the special high-current, deep-discharge "leisure" type, SLA batteries are intended for low-current applications (usually with trickle charging) like alarm systems or emergency lighting. I use them only for our show demo boards and as weights in the workshop. My models are all fitted with either NiMH or LiPo battery packs; in my book the only use for lead-acid batteries in a model - as Calimero says - is as ballast.
Suit yourselves - as someone used to say.
DM