I can't easily find a nice and comprehensive comparison, but it seems like he was right even with Pearl Harbor included. From [1]:

> Total U.S. military deaths in battle and from other causes were 407,316. The breakout by service is as follows: Army 318,274 (234,874 battle, 83,400 nonbattle),[292] Navy 62,614,[292] Marine Corps 24,511,[292] and the Coast Guard 1,917.[570][320]

I don't know what the denominator looks like for those various forces, but 5x as many Army deaths as Navy seems to corroborate your grandfather's intuition.

[1] https://en.wikipedia.org/wiki/World_War_II_casualties#endnot...

Edit: From [2] it seems the Army was a bit over 2x the size of the Navy, so still about 2x the rate of deaths.

[2] https://warfarehistorynetwork.com/us-involvement-in-wwii-how...

I've read that being a civilian in the Merchant Navy was more dangerous than being in the armed forces.

We lost a lot of those ships.

My grandfather joined the Navy at 17 in 1943 and never spoke about it. I assumed when he was alive he didn't do much in the war but in researching his ship it was the complete opposite.

The Battle of Leyte Gulf was the biggest naval battle in history. If he was in the pacific I am sure he seen some crazy things.

My grandfather was also the Battle of Leyte Gulf. He was on the Kitkun Bay. He didn't start talking about it until maybe 2000 or so, then he opened up a little. He got sort of obsessed with reading history books about his own battles. There's still a lot we'll never know though.