No death certificate found when applied for.

As said, if no body found, no death certificate is issued. You may have to go thru the process of declaring him dead if it wasn't done prior. Did MIL remarry? To do so, she had to declare him dead.

You may need a lawyer for this.

Your husband's family probably should not have waited over 50 years to get a copy of the death certificate. An already difficult process will now be 10 times harder.

Your FIL disappeared and is pressumed dead from drowning however, no body was ever found. Is your MIL still alive to answer any questions?

I personally have not had good luck trying to get even a copy of a death certificate online through services like Vital Check... I think they are somewhat of a scam. But that's my opinion.

Here is some useful information to get you started:

https://en.wikipedia.org/wiki/Presumption_of_death

"If there is not sufficient evidence that death has taken place, a legal declaration of such may take longer, as simple absence does not necessarily prove death. The requirements for declaring an individual legally dead may vary depending on numerous details including the following:

- The jurisdiction the individual lived in before death
- The jurisdiction where they are presumed to have died
- How the individual is thought to have died (murder, suicide, accident, etc.)
- The balance of probabilities that make it more likely than not that the individual is dead

Most countries have a set period of time (seven years in many common law jurisdictions) after which an individual is presumed dead if there is no evidence to the contrary. However, if the missing individual is the owner of a significant estate, the court may delay ordering the issuing of a death certificate if there has been no real effort to locate the missing person. If the death is thought to have taken place in international waters or in a location without a centralized and reliable police force or vital statistics registration system, other laws may apply."

and

"If someone disappears, those interested can file a petition to have them declared legally dead. They must prove by the criteria [in this article] that the person is in fact dead."

Please read this wiki article. Look under your country (US) to get general guidance. Then I would start by contacting the county government offices where his disappearance (drowning?) was first reported — assuming there was a missing person report made in the first place. 1972 predates digital files and computers by decades, so hopefully someone in some office will be able to track something down, if it exists.

I wish you success in getting the declaration.

Lost at sea is lost. It isn't dead. In order for a death to be declared a legal action needs to be taken, usually after a certain time frame (often 7 years) according to the state.

See an attorney to do the process of declaring the death and filing a death certificate with the state according to the state's legal requirements.

Your husband might have to initiate a death certificate process. But it’s very possible you are looking in the wrong place.

What entity or municipality declared him dead?

Without a body, there may not be a death certificate.