Arno,. You are correct, the Divinycell won't rot if water gets in. For a boat with a teak deck that has fastener holes I would prefer the Divinycell since with that many holes water is going to get in.  I have been in the boat repair business for more than 40 years and I have dug out a lot of rotten plywood and balsa core and you are correct it is a huge nasty job to do right.  The Amel design thankfully has very few deck and cabin penetrations so it is not that big of a job to decore each and every hole, fill with epoxy, redrill and seal.  From that point forward if a fastener leaks the water cannot get into the core.  The increase in compressive strength of the balsa will create a more Ridgid structure and the increase in the bonding to the fiberglass skins (assuming the layup was done correctly) also adds strength.  So this means that with a weaker core you probably need to to build the boat a bit heavier to have the same strength.  Are the HR's heavier than a comparable Amel?  I have worked with the Divinycell on sailplanes and I will say that it the the best foam type core that I have worked with.  Some of the planes were 20 plus years old and the core was still healthy.  I have seen many other types of foam used for core material break down.   So to distill yes on a boat where I feel that I can keep the core dry, I would much prefer to have balsa core such as my Amel.  On a boat where there is a high probability of water intrusion such as a deck or cabin with a lot of screw holes, a non organic core such as Divinycell would be preferable.  Best to you.