The current tune of searooms is quite harsh (too harsh in my opinion).
Upon entering first square of sea, with failed swim you instantly
became dehydrated. Especially bad with demon on.
One solution would be to add another (much easier) swimcheck for
holding ones breath. Upon failing swimming you'd perhaps take some
suffocating damage, but you wouldn't become dehydrated right away.
Another solution would be just to lose the contents of stomach, but
not all the liquid running through veins. Meaning you'd only lose some
fixed amount of water from your body, not all.