More money and staff are needed to protect wildlife and habitats in the waters around Wales, the assembly's environment committee has said.
A report into the management of Wales' 132 marine protected areas - covering more than half of Welsh seas - said each should have a dedicated officer.
The AMs also want assurances over legal protection and conservation funding for Welsh waters after Brexit.
The Welsh Government said it is working to have a resilient marine environment.
Marine protected areas (MPAs) have been created and are supported under a range of domestic and international initiatives.
The report calls on the Welsh Government to show leadership on marine protection, by providing staff and resources to improve public awareness, research, monitoring and enforcement.
Mike Hedges, Labour chairman of the committee, said: "Welsh waters are home to some of the most biologically diverse habitats and species in Europe and 50% is protected, which means that certain activities such as fishing and wildlife trip-operators are regulated to prevent environmental damage.
"There are a great many different levels of protection, designated under a range of laws, but this designation has not resulted in better management, with voluntary codes of conduct not always being adhered to.
"Simply creating Marine Protected Areas is not enough; the Welsh Government must also ensure they have the resources necessary for the management, monitoring, surveillance and enforcement that is required."
Mr Hedges said it was "vital that we in Wales can design policies that are tailored to protect our seas" after Brexit.
But he added: "There must be no reduction in, or loss of, protection.
"Importantly, this will require funding and we need to know that it will be available in the future."
A Welsh Government spokesman said: "We welcome this report and will respond to its recommendations in more detail in due course.
"We are committed to working with partners to ensure we have a resilient and sustainably-managed marine environment which supports a rich diversity of wildlife, is highly valued by local communities and is able to sustain important activities such as fishing, tourism and recreation."
Gill Bell, from the Marine Conservation Society, welcomed the report, saying: "We have been calling for greater accountability and responsibility from the Welsh Government for many years.
"This can only be addressed if marine conservation is prioritised.
"If not, we risk our special areas of sea becoming mere paper parks, losing wildlife and the vast benefit that our seas deliver for people in Wales."