Don't use rubbing compound!!!! Whilst it reaps great results it should be used only a handful of times on a boats hull/deck in its whole lifetime.
Get hold of a Fibreglass and Metal polish called "Fritz" - try google - this is great for all the fibreglass on your boat and metals. Its a great polish that reaps awesome results
its not super cheap but it does a great job and a tiny amount goes a very long way... a bout 5ml will do your whole transom no problem!
The black crap round your transom is from your exhaust fumes... its particuarly bad as you are breaking the boat in and will get better in time (lots of oil being burnt).