Complaint number one is very valid....security should have physically removed the "boom box" on the very first night.
The slow draining shower issue should only be addressed IF you called maintenance and they came and were not able to fix it. That will be the first question GS will ask.
I never went to the Aft pool, but on most Carnival ships it is the Aft hot tubs that are adult only.....not the pool itself. IF the sign was on the hot tub wall...that was for the hot tub.
I would write a short, well worded letter....starting with the good things that happened and then add...."however the reason for my letter is..." and note the complaint that Security clearly did not do their job.
Then I would only note the slow drain IF you filed a maintenance report and asked it to be fixed.
Not sure I would note the hot tub issue unless you are positive the "adults only" was for the pool AND the hot tubs....since on most ships..it is the hot tubs only.