I Sink!

Question
Hello, I thought I would ask you this question as you are the diving expert.

My girlfriend recently forced me to go swimming!  It's not something I've done much before, but it would be useful to be better at it.  My problem is my bottom just sinks and drags me down! I tried just floating on my back, all relaxed, even holding loads of air in my lungs, and always, my arse just sinks!  If I keep straight my entire bottom half sinks, then drags the top half under.  I have a very muscley backside, is that the problem?  If I had more fat would I float better?  Is fat more floaty?  Im no stick insect, but I just wonder, are some people just made in a way that does not allow them to float?

I hope you can help, thanks for reading my long meandering questions!

Answer
Dan--
Yes you are one of the unfortunate ones who will have a difficult time floating.  Being very lean and muscular makes it very hard to float.  Fat is definately more "floaty", which makes it easier for chubby people to float.  You can still swim, but just floating without some sort of flotation device is not real easy.
Hope this helps,
George