When choosing an outboard motor the #1` question should be "Is this motor wanted for water-skiing". If so one needs a motor with a serious amount of "grunt" and adequate reserves of strength and power. If one is not too heavy it will be possible to ski with 25HP but beware! The reason is that many 25HP motors weighing about 80 pounds are really just "hopped-up" 15HP motors. Once one moves up to approximately 30HP the motors suddenly get 50% heavier as all the parts are strengthened. Amply sized bearings and bevel gears are most important in this respect as some brands of motor will not stand up to skiing even when they are new. The big Mercuries are absolutely impeccable given a modicum of attention but some brands of motor can strip their gears after just a few days of skiing. (e.g. a Terhi 25 that I owned from new chewed up three gearboxes) The delightfully light motor was actually based on a 15HP model but the powerhead had been bored and stroked. Pity that the gearbox bearings weren't uprated at the same time. Cost me £s£s£s. With the benefit of hindsight a Mercury 20HP which was actually 23.5HP would have been a much better buy and would have probably stood up to skiing OK. What some engine designers overlook is the strain caused by a skier crossing the wake and cutting in hard. When this manoeuvre is performed the engine revs get pulled right down hence the powerhead is putting out the maximum torque that it is capable of . This coupled with a fine pitch ski-prop will "blow" the bearings and gears in under-engineered smaller motors. Such failures cannot be prevented by maintenance, bad design is bad design. Having said that it is sometimes possible to get service from under engineered machinery by fitting premium grade bearings instead of generics.
Great fun though water skiing and the closest most people get to walking on water!
If motors are only wanted for inland waterways which have 4 knt limits almost any motor will do although 2 cycle motors are now out of favour in many areas.
Nowadays the main causeof "failure" is the failure of the motor to be there. Now that is a really big problem that only goes away after one"swallows the anchor".