I posted a notice almost identical to this about 15 month ago (engine was a couple of months old at the time). I got two replies from people with this problem. When I tackled the dealer (Poole in Dorset) about it he was sympathetic and confirmed that he had a few customers with this problem and despite significant efforts on his part he had not been able to get to the root cause of it. His solution was to supply Suzuki stainless shear pins (he was also a suzuki dealer) which solved the problem, and since then I have had no trouble at all. One of the people who replied to my original mail was told by his dealer that his warranty would be invalidated if he fitted stainless pins so was left with the unhappy situation of no warranty, or an engine which let hime down periodically without warning. Needless to say he was not very impressed.
As I said my dealer was sympathetic, and agreed to honour the warranty with the stainless pin (he supplied me!) fitted. The problem it seems is that whilst Honda look closely at warranty claims, and change designs where there is clearly a design problem (eg the 2hp engine), shear pins are not covered by warranty, thus the continual failing of this part does not get flagged back to Honda as a design isue. My dealer had written to Honda UK on more than one occasion about this without success.
My advise would be to approach your dealer (assuming it's under warrenty) and ask him if he has any other customers with this problem. He almost certainly will have, and try and persuade him to honour the warranty with a stainless pin fitted.
The other reply to my original mail also ended up fitting a stainless shear pin.
I know it's small comfort, but you are not alone in this.
I have a Honda 5, it (she?) has never broken a pin except by my own fault. this includes grounding the prop on occasion. my only tip is to check your motor's idle speed, too high and it may be enough to break a pin.
Sorry for such a late reply. I had exactly the same problem. The dealer was quite stroppy on the phone and claimed never to have come across such a problem. I decided that it would be better to risk damage to the prop (an observable danger) than to be left without drive on a lee shore and falling tide, as happened when I nosed into a slipway to drop a crew member and then broke the pin going into reverse (a quite unexpected danger). The shear pin broke three times before a realised it was not my fault. The motor is an auxiliary on a 20ft fin keel sailing cruiser so I rarely endanger the prop.
I made myself some 5mm pins from threaded stainless steel, feeling that the thread cuts would weaken it a bit.
Funnily enough when I went into the dealers again he gave me a notched 5mm ss shear pin (Suzuki?). I must say I feel much happier now. I continue to try to avoid striking anything with the prop.
Finally, I do take the point about idling speed, but it has never seemed very fast in fact and I'd rather have the the sronger pin.