Ominous Portent
They knew there was an underwater missile complex being constructed under top-secret conditions and they knew the general location but not the co-ordinates. They didn't want to destroy the complex only to be able to control it at a future date. To control the missiles the plan was to copy the four fail-safe keys that would be delivered to the complex by selected delegates at the commissioning ceremony.* With those copies in their possession, at any time in future if they took the complex, with some other dealings, the missiles could be launched at their command.
The top officer of the Seaview was to be a delegate to the commissioning ceremony and would have information they wanted. The Seaview was Admiral Nelson's vessel. Since this boat was small with a complement of only 125 men, was a submarine therefore easy to hide, and was a civilian owned vessel, only occasionally commissioned to the military, Seaview's delegate would be the easiest one to get the information from.
Someone else wanted to talk with Nelson too, wanted that very badly indeed. He had thought about a little talk with Nelson unceasingly since his brother's death. A death so unresolved, no body, no grave, just the unsatisfying we-regret-to-inform-you pronouncement. Just a little talk …
* Doomsday