Hi Steve and congratulations on your purchase I am jealous you are going to lembeh!!
Onto your questions out of the box the s100 don't do ttl in manual and manual is what you need for macro
If you look around on google for CHDK ttl canon s95 you should find information on a script I have adapted for the canon hack that will enable ttl in manual
If you don't manage or your strobe doesn't work in ttl with the s100 your starting point is the camera without a diooter and for super macro the subsee +5 unless you have Pygmy seahorse and you want to try the +10 this should also make it easier to focus especially if you don't have a focus light that I warmly recommend
Settings
ISO 80 after all is a small sensor camera you want to keep noise to the minimum
Aperture f8 the subsee are powerful and bring high magnification without you may keep to 6.3
Shutter 1/250 or higher as at high magnification the minimum movement result in motion blur this still gives you some blue background against water otherwise 1/500 for black background
Zoom wise I recommend working at full zoom and finding the working distance otherwise you may end up on top of the critters
I recommend you practice on land with the diopters to determine the working distance at full zoom as that will make it much easier to focus basically once you see it magnified on the LCD you shoot to that respect set the magnification are to small
Happy shooting