Pete,
It just depends on what you're calling a 'sealed' battery. The usual 'sealed' lead-acid battery (like an automotive battery) isn't really sealed. It isn't easy to get into them (sort of), but they have to be vented or they will explode (or more accurately, rupture the case). If you're talking about a 'gel-cell' battery, then they won't explode unless you really do something silly like crack/puncture the case. The aren't 'position sensitive', you can mount them up/down/sideways with no problem. No openings in the case at all, so nothing will leak out, hydrogen gas or electrolyte. Of course, if you get them too hot, then sure, they'll cause problems.
Lead-acid batteries are not 'good' for models because of the vented hydrogen, and more importantly, because of the sulfuric acid content of the fumes. Causes a 'meltdown' of whatever the model is made from and you end up with a disolving boat. (Also causes @#$% with the 'life' in the pond where you're sailing.)
If your's, or your friend's gel-cell exploded, then you/he must have been doing something really 'odd'...
- 'Doc