Hi Mirko
Yes I remember the trip, cool people, the weather was great and the Manta's were back at Koh Bon.
Things are busy at the moment with students so I have not been doing any UW photography.
Sounds like you have a great trip planned and Cocos is high up on my list of places to visit and a good camera is a must.
As for the camera I would look further at the D5000. as the responsiveness and custom controls of a DSLR is much better than a P&S. The only thing that concerns me is the D5000 has a limited lens choice like the older D40/D60 is it does not have a autofocus motor and therefore the number of lens is limited for autofocus, essential for UW photos.
I found a very good review here
http://www.dpreview.com/reviews/nikond5000/As for the housing, yeah Ikelite is a good choice and I am very happy with mine, though it is a little heavy and bulky as you have already seen, but that is the only complaint I have.
The other good thing is that if you get the DS-160 with the IKelite housing you can use TTL via a cable from the strobe to the camera. TTL works great for closeup subjects and I find it about 95% accurate and automatically adjust the power output of the strobe give the correct exposure and can easily be adjusted manually to increase or decrease strobe power.
This function is not possible with the G10 and all strobe power adjustments would have to done manually or with an external TTL converter. More expense and more weight to carry if you ask me.
Another thing you need to think about is how much will this weigh (excess baggae on planes costs a lot now and my kit weighs in at around 20kg), what lens can you get (I use alot a 60mm for macro and fish portrait and a 12-24mm wide angle for seascape and large fish). You will also need a different port for the housing for each type of lens you use, eg a flat port for my 60mm lens and a dome port for my 12-24mm lens. These are around $200 each.
I hope this helps and if I can be of any further help let me know and I look forward to seeing you next season.
Spencer