What am I?

I have a special bubble
in my bladder
to help me to detect
if my posture’s not erect –
and straighten it, correct;    
adjusting it in volume, size,
enables me to sink or rise;
I’m glad my independent eyes    
mean I can look both left and right –
and gain from such wide-ranging sight,
because I can but slowly swim
(I do taste bitter, don't appeal
to those who want an easy meal);
and though I am a ‘him’,
I have a kangaroo-like sac
to take her eggs and fertilize,
gestate and baby-sit until
with pushing, squeezing, out they spill.
Some say I look quite like a horse,                        
but I’ve no legs to race the course:            
see me underwater sail,
or gripping with my grasping tail
weed in the sea or aquarium tank,
bobbing with my bubble
and our brood in the pouch on my trunk.