Is there any technical reason for Garmin DIve watches using a different charger than other watches?

I do not know for sure, but I would tell that it is to assure the water-tightness. It might be challenging to properly seal the pins of the smaller connector, under such high pressure. It should be also easier to clean the bigger contacts, remove the salt deposit from them, and to avoid short-circuits between contacts when wet, thanks to the larger gaps.

trux said:

but I would tell that it is to assure the weathertightness

Yes, it would make sense that the different style clip charger is to ensure the 100m Dive rating (Mk3 is now 200m).

The same clip charger is on the Mk1 and Descent G1 series as well.

err, sorry for the typo - should have been "water-tightness" 

trux said:

err, sorry for the typo - should have been "watter-tightness" 

all good, we get what you mean

I doubt sealing is the reason. The other watches like the Fenix are also rated to 10 atm (I had the Fenix, then the Mk1 and Mk2i). My purely speculative guess is that it is for a more secure charging (and connection to a PC for USB/charging) when the watch is on a dive boat in ocean conditions, for example when jostled around at night in tight quarters, complicated messy conditions, as most dive boats are, having a secure clamp is more secure than the smaller Fenix-style connector. Just a guess. 

yes, thanks... while I understand that water-tightness is fundamental I tend to believe the main reason may be to make it easier to clean, removing salt and other things...