In LEN European Masters OWS Championships they requires minimum time marks (which are quite permisive) by pool or open water (different times)
The problem is how to check this, because it´s really hard in a non-pro age groups, in addition the number of participants swimmers is usually higher than 3000. I know many people that have registered without certificate that marks, and they had no problems, because all of them have official license, have a minimum or self-responsability, and organization can´t refuse your registration.
In Navia, we have local old man (72 years old) that loves swimming and participates in our OWS race every year. He have official license, and swims in 1100m. race. We are a bit concerned about him, because, as I said, risk always exists. He always finish in last, but I must say many spectators are waiting him in finish and the bigger cheer is always to him.
From our position, as Organising Comittees, we must provide the correct & suficient security measures, both in water (boats, kayaks, surfers) as in land (medical area/tent, proffesional doctors, ambulances). Our other job is to verify the correct license of the swimmer or decide to formalize a private insurance (that requires a medical certification).
If you still feel it necessary to take any action, you can fix in your rules an "out of time limit", such as professional races. When the limit is exceeded, all swimmers that are on course yet, are forced to retire and are picked up by security boats. I'm not very keen on this measure, because we must encourage participation and many of those who swim do so better themselves and just achieve took finsh platform is a challenge for many. However if security conditions recommends, it is necessary to aply. Safety must be paramount