I can tell you this first hand... for I have lots... ehhh... LOTS of experience in this...
Coins...
They do not sink quickly most times.
Growing up in water... a pool and a dock in my back yard... I practice my breath holding skills... all started with my dad tossing a handful of change into the pool... which I loved and had him do constantly... mainly cuz I got to keep the change... but anyway...
Arrhh... basically... he would count to three then toss his pocket change into pool and on three I could launch into pool...
I was at bottom before most had made it half way down... so the change would be raining around me.
My deal was to see how much I could get in that one breath... each time trying to make / beat a record.
Sorry... felt the need to share... and quite frankly its been so many moons ago ... I forgot about it... so it was good memory.
PS... I might add... my breath holding / swimming abilities earned me many nicknames... the first being "fish" at age sheesh... I guess when went off diving boards at 2... um somewhere around then.
I quickly grew outta that nickname though. :P