I used to use a 3 hp 2 stroke on a bigger heavier SIB than that. I did many miles on the sea, inshore, with diving gear. It was easily big enough to push the boat at maximum non-planing speed.
Very roughly speaking, a boat has a maximum speed that it can go through the water. The only way to exceed that speed is to use enough power to get it up "on the plane" which means skimming across the surface rather than ploughing through the water.
Therefore, for the specific use that you have described, you would be OK with the small engine.
However, there are many variables to take into account.
One is the price, of course. Another is the weight to carry when setting up or putting away the boat.
Another is noise. A 3 hp at full throttle will be louder and more intrusive than a 6 hp at half throttle.
A heavily laden boat (crew/passengers/fishing tackle or dive gear) will sit lower in the water and require more power to reach maximum speed than an empty boat. And so on.
The only problem with your choice as described is that it would limit you to that. If you genuinely want to do nothing except motor half a mile to your yacht, or to a fishing spot off the rocks, or use the boat as a sun bathing platform or swimming platform in a sheltered bay, a smallish engine will do the job perfectly well.
If you might want to spread your wings, go a little further, keep your options open, cope with worse sea conditions etc. then it would be wise to build in some reserve by buying a bigger one.
Then the choice becomes between the biggest you can get afford and carry an integral tank (takes up less space in the hull) or going up a step to a 9.9 or 15 with a remote tank.
Faster and more powerful is not always better. It depends what you personally want to do with the boat, how often, and how much you want to invest.
I've had some of my best days out poddling about with an egg whisk of an engine, taking the time to watch the wildlife, admire the scenery, and enjoy the trip. There are many merits to having an engine you can easily carry between the car and the slipway.
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