Was the explosion intentional? I thought they were hoping it would float for recovery but if not I makes sense to sink it.

It's more of a financial question, do you want to shell out for some chance at it. And Russia and China can build their own engines, SpaceX is very good but not like a century ahead good.

(If anything, I'd be more worried about North Korea or Pakistan getting their hands on stuff…)

Either way the risk is the shit that's left floating, if they don't fish it out it'll randomly wash up in Madagascar or so…

They splashed down in the Indian Ocean, not the Gulf of Mexico.

Also noone mentioned submarines, debris can just float and drift around (sometimes even at some depth below the surface).

Starship landed in the Indian Ocean, not the Gulf of Mexico.

I mean, yes [0]. It's probably the main reason SpaceX went to effort to recover [1] all the engines of the previous booster (IFT-4), which landed in accessible, shallow water in the Gulf of Mexico. The Raptor engines hold valuable secrets, particularly to China who are trying to clone a lot of SpaceX things.

The CIA did something very similar to this in the Cold War [2], though they used a boat instead of a submarine.

[0] https://en.wikipedia.org/wiki/Akula-class_submarine#Operatio... ("In August 2012, the news media reported that another Akula-class submarine operated in the Gulf of Mexico purportedly undetected for over a month, sparking controversy within U.S. military and political circles...")

[1] https://old.reddit.com/r/SpaceXLounge/comments/1fvdekt/more_... ("More images from B11 recovery + new info "26 of the Raptors have been recovered but they are trying to get all 33")

[2] https://en.wikipedia.org/wiki/Project_Azorian