A million seagrass seeds are being planted as part of Britain's largest project to save the "wonder plant".
Experts say seagrass helps tackle the effects of climate change by absorbing carbon dioxide faster than trees.
But up to 92% of the plant may have disappeared from the UK's coast over the last century, research has found.
Work has now started on lowering the seeds onto the seabed off Pembrokeshire to create a new 20,000 sq m (215,280 sq ft) meadow.
Scientists hope it will also help boost fish numbers and support marine wildlife.
Seagrass, which is found in shallow waters of coastal regions, has been declining globally at a rate of about 7% a year since 1990.
That is a result of long-term development of our coastlines and pollution of the sea, according to project leader Dr Richard Unsworth, of Swansea University.
"It is not that we can blame one person, industry or organisation, it's the growth of a population around the coast," he said.
"Planting seagrass is an opportunity to reverse that loss and start to kick into action a recovery for our seas around the UK."
World Wildlife Fund (WWF), Sky Ocean Rescue and Swansea University say the underwater plant is key to reducing carbon dioxide - a gas which contributes to global warming.
They hope the 4.9-acre (2 hectare) project at Dale Bay will also provide a nursery for young fish and a habitat for invertebrates.
"It's incredibly productive and just sucks carbon into the sediments, traps particles that are locked there for millennia," said Dr Unsworth.
"That means that carbon dioxide is not in the atmosphere."
Last summer, 750,000 seeds were gathered from sites around the British coast and stored at the laboratories in Swansea University.
The seeds have been transferred into small hessian sandbags and lowered onto the seabed.
Another 250,000 seeds will be gathered later this year and added to the meadow in November.
"We see seagrass as this wonder plant because of its ability to fight climate change, to help fish stocks, coastal communities and livelihoods," said Alec Taylor of WWF.
"We need to expand hundreds of thousands of hectares of seagrasses, saltmarshes and other coastal ecosystems to avoid some of the damages from climate change."
Why is seagrass important?
- It takes carbon from the atmosphere up to 35 times faster than tropical rainforests
- It accounts for 10% of annual ocean carbon storage globally, despite only taking up 0.2% of the seafloor
- It protects coasts from coastal erosion
- It is a habitat for many types of fish like cod, plaice and pollock
- It produces oxygen
- It cleans the ocean by absorbing polluting nutrients
Source: WWF, Sky Ocean Rescue, Swansea University