Hiya
Up until a few years ago, all outboards were two stroke petrol driven motors. You do get the odd diesel driven outboard, but they're very rare. When four strokes came onto the market, they revolutionised the boaters world. They were lighter on fuel, no, CONSIDERABILY lighter on fuel, roughly 30-50% lighter on fuel to a similar sized two stroke motor. They didn't use two stroke oil, so weren't smelly and didn't bellow out copoius amounts of white smoke!! (the bane of all of those who suffers from mal-de-mer/"sea-sickness"). They were more environmetally friendly and they were whisper quite. On the downside: they're EXPENSIVE, are much heavier than a two stroke motor of same power and the servicing costs are expensive.
The two stroke tecnology has also moved in leaps and bounds over the past few years, in response to the four stroke tecnology. You new generation two strokes are virtually smoke free, but still use two stroke oil. The greatest leap forward in the two stroke field has no doubtly been the direct injection fueling systems. They made the two strokes now almost as fuel efficient as their 4 stroke counterparts, but also just as pricey!! The only other advantage was that the new generation two stroke fuel injection models have is that they're lighter than the 4 stroke but still heavier than the old two stroke.
In some countries there is talk of eventually phasing out the old two stroke models, due to emmision egulations.
For your application, a oldish normal caburetted two stroke would work PERFECT. Light in weight, cheap to purchase and reasonably light on fuel. They're also mechanically very simple!! Low on maintance and generally trouble free!!
There are many marques, each with their own followers:
YAMAHA - regarded by most as THE engine on choice. I've been a yamaha fan all my life. The fact that 90% of our commercial fisherman use YAMAHA'S bears testament to their reliabilty. Both their 2 and 4 stroke range's are superb.
Mariner - the early mariner models, especially the smaller 20-50hp motors were actually built by yamaha. You can still see on the older motors, the covers and all the stickers says Mariner, but the vin tag shows built by YAMAHA rofl Haven't heard many bad things from them.
Mercury - some love them some HATE them. Their opti-max (2 stroke fuel injection) has been called OPTI-BOMB becuase of them blowing up!! rofl Seriously though, some users swear by them and others swear at them!!rofl Since i've never owned one, i would know!!
Suzuki - the early two stroke models had a bad reputation here for corrosion problems here in South Africa. I know the American markets regard them very highly. Maybe we recieved a different motor? They did work VERY well in our fresh water applications!! Anyways, the 4 stroke Suzuki's are PHENOMINAL. They're collected basically all the awards a outboard manufacturer can get!! Even the Johnson/Evinrude (OMC) 4 strokes are built by Suzuki and merely rebadged!!
Johnson/Evinrude (OMC) - i'm a bit biased here, as my very first boat had a Johnson that was very old. I had it rebuilt, but it was a major head-ache from day one. How-ever, it was a OLD motor, so that shouldn't be a reflection of the make on a whole. They built OK motors, but a few years back, BOMBADIER got involved with them and they produced the FICHT motors. Fuel injuction two stroke that was as light on fuel as a 4stroke BUT it required no maintance for 2 years (or 300hour i think!!) Once again, most owners LOVE them and i've heard very little complaints about them.
You do get other makes, like Tohatsu, Honda, Yanmar, etc but i'm not very familiar with them. Remember the above are simply MY OPINIONS!!!
Regards
miles