The Yamaha is a 1.8L block. Obviously a 1.5 making 100hp is going to be working harder than a 1.8 making 90.

We have a 70 Zuk that's approaching 1000 hours and has been bulletproof. Literally zero issues, all it's ever needed is petrol and servicing every 100-120 hours. I will say it is much louder than my friends Hondas, especially if you give it some throttle, and produces considerably more vibration.

While its true that a broken cam belt can destroy an engine........ so can a broken chain . A  belt on an outboard is relatively easy to get at   not like most cars   so a lot of the labour bill is reduced .

 A cam belt can be inspected without any major work to get an eyeball on it  ditto for the tensioner and idler pulleys 

 Cam chains are much harder to inspect  but they last much longer. They also involve a lot more in the way of tensioners and guides .... they are not as bullet proof as some think  . If the tensioner or the guides fail  your in as much 5hit as a failed belt. chain guides do wear and in some cases disintegrate to the point that the chain gets loose  and starts hitting the inside of the engine cases  and/or jumps a tooth or two  and then we get valves hitting pistons . This is more likely in an outboard than a car as the outboard chain runs horizontally   and a car chain runs vertical. 

It took me 1/2 hour to change the cam belt on my honda outboard, and not much longer to do my Ducati ( two belts )  Changing a cambelt is no more technically challenging than assembling a Kinder surprize toy ........just getting at the thing can be an issue , best take it to the shop if patience and skill dont rise to the occasion .

 Both my cars have the chain   and Im quite happy about that  but the Rav4 has 180,000kms on it so I an soon to take the top cover off the engine and have a look see ..... I am expecting it to be OK as the car has been well serviced ( by me  )

Personally I would go for the belt .... specially on an outboard . I will say tho that being able to do the work oneself  will influence any decision . I will also say Im nearly at the end of doing cambelts for friends cars  .