You would need to check whether the tubing and the outlets were suitable for very high temperatures. The hot air coming out can melt normal plastic. The heater unit itself does get hot, but has a cover over it to prevent burns.
The
exhaust is the most dangerous area. normally this is made of flexible
stainless steel (similar to that used in exhausts) covered by abt 0.5" asbestos substitute and then covered by another tube. It still gets hot enough to make you keep your fingers clear of it!.
Impossible to tell you how long it would take to do the full install cause it depends on the
boat.
One of the most important
parts of the install is the
exhaust, as it needs to be clear of the
water when in use, and also to have sufficient of a swan neck so that
water cannt get at the heater via the exhaust. It also needs to be less than 6 ft long.