The pledge is among a series of commitments being made at the United Nations COP26 climate conference in Glasgow, where leaders and negotiators have gathered to keep alive a receding target of capping global temperatures at 1.5 degrees Celsius (2.7 Fahrenheit) above pre-industrial levels.
Among the deals so far is a pledge to end deforestation by 2030, and cut methane emissions by 30% by 2030 from 2020 levels.
Scientists and activists have called on countries to also recognise the link between oceans and climate change, arguing that sustainably managing the seas can help better regulate the Earth's climate.
US climate envoy John Kerry announced the United States would become the 15th country to sign up to the oceans pledge, which is endorsed by other ocean-dependant economies including Indonesia, Japan, Kenya, Chile and Norway.
It calls for greater investment in ocean-based renewable energy, decarbonisation of industries, and further research.
But the statement made no mention of ending massive annual government subsidies that prop up activities such as industrial fishing, a major driver of over-exploitation of the seas.
Greenpeace, an environmental NGO, called the declaration "weak".
"We need to see action to create a network of ocean sanctuaries that cover at least 30% of our oceans by 2030," said Louisa Cason, oceans campaigner at Greenpeace UK.
"We need areas with zero commercial extraction, where nature and fish populations that fisheries depend on can recover and thrive."
Two-thirds of the planet is covered in water, and oceans absorb both heat and carbon dioxide and distribute it across the planet. But with greenhouse gas concentrations at their highest levels ever seen and temperatures warming at an alarming rate, marine ecosystems are struggling to keep pace.
Dawn Wright, chief scientist and oceanographer at ESRI, a US mapping data company, told Reuters in an online interview that understanding the relationship between oceans and climate change is crucial for delegates at COP26 to be able to table a plan to sustainably manage oceans.
"We are currently severely undercounting carbon emissions that result from human activities in the ocean. Things like trawling by fishing fleets, activities that disturb the seabed.
"We must include oceans in how we account for emissions and pollution, and I hope COP26 will recognise this problem."
