Sometimes, the potent spirits guiding the evolution of their chosen species had a grand scheme in mind that shaped their decisions, or perhaps they reacted to short term problems to get results. Sometimes... sometimes it seemed that nobody was at the wheel.
The crusters lived a fairly blessed life - grow, crust, reproduce, get eaten (hopefully in that order), offspring repeat. Yet they remained in the shallows, unable to spread beyond the plentiful carbon dioxide and light. Spreading into the deeps would require more efficient photosynthesis and better dioxide extraction. Spreading to land would require a more stable structure.
Instead, something nonsensical happened. The crusters developed a mutation that spent some of their excess of energy not improving their lot, but instead expending it on the useless production of atmospheric hydrogen and oxygen. It was indeed the case that oxygen was needed for respiration (photosynthesis simply produced oxygen for a plant's respiration, the excess it exuded was a by-product), but splitting water required energy that could only be produced as a result of photosynthesis regardless.
Nevertheless, hundreds of little crusters wasted their energy, bubbling away.
Cruster Evolution: Develop capacity to hydrolyse, splitting water into atmospheric hydrogen and oxygen that can be used in respiration. Excess hydrogen escapes from plant pores as small bubbles.