If we're talking billionaires wouldn't they have the resources to build/modify one of these luxury subs to carry a missile? This company is conveniently located in Seattle… (+Paul Allen's personal sub)

https://www.browniesmarinegroup.com/pdf/Triton/US_Submarines_BWMG.pdf

https://www.strategypage.com/htmw/htsub/articles/20070711.aspx

https://www.richandloaded.com/assets/most-expensive-submarines-paul-allens-yellow-submarine/