To those saying he died of blood loss from the shear impact of the bullets and not penetration than it makes no difference. There's bound to like 5x times more guards at Mariejoa. Hell, its probably harder to get to the celestial dragons than infiltrate impel down. They are people protected by the admirals so of course they're going to be guarded heavily. Hancock said (or implied) that he attacked them head on. I don't know i have to reread but i'm almost certain Hancock said something that confirmed to us that FT was not sneaking around and it was head on. Hancock said she ran for her life so FT was NOT leading them.
A possible explanation though…could be that FT was in his crazy fishman berserk mode (you know the mode Arlong was in after Luffy really pissed him off). I don't see that making much of a difference though. Maybe Dragon unleashed a crazy storm on Mariejoa that day? That would be an excellent explanation...wet gun fire and Tiger is a beast physical fighter. People stop trying to justify it as of what we know now
--- Update From New Post Merge ---
@kouch_lee:
We should check what a Plot-Hole is. FT being defeated by a squad of marines ain't no damn plothole. Whitebeard got killed by guns too, remember?
And just to add some info to this discusion: remember when Luffy entered the Marineford war and charged alone into battlefield? 2 of this so called "fodder marines" injured him. Luffy himself admitted that everyone was damn strong on that battlefield.
I think the marine that FT punches away is a coated marine, we know these are stronger than non-coated ones. There are a bunch of stronger-than-usual marines in there.
Also, just look at Jimbe, Arlong, Aladdin and co. after rescuing FT. They're really injured, ALL of them. It wasn't an easy battle defeating those fodder marines, and maybe they didn't even defeat them, just escaped.
One more? As Urouge said, FT probably uses weapons when engaged in sarious combat. He looks to me like a brute, pure physical strength, but not much finesse. Probably that's why he uses weapons in combat, weapons that he didn't have this time with him.
And last but not least, just remember last chapter. He defeated another Rear Admiral with one hit. FT ain't no joke. He was caught off guard, without weapons, by a group of probably pretty strong marines, and he could've survived.
I'm not saying this was the BEST way to kill FT; hell, I would've killed for a Kizaru vs FT, or even Strawberry vs FT, a straight-on fight. But that wasn't the intention of FT character, to show how "badass" he is, but to show his inner struggle with trying to love humans but ultimately fall to his own hate for them. I think it's a far, FAAAAR more important issue right here, and for me, it was masterfuly shown.
Yeah well Fisher Tiger wasn't old as hell, didn't have health problems, wasn't shot by 400 cannons and bullets, wasn't magma fisted, wasn't being amushed CONSTANTLY, and shouldn't have had this mind distracted. WB wasn't killed by guns. It was his time and i'm pretty sure it was of a broken heart AND AGE. He could have easily quaked BB but it was his time. I don't see Strawberry himself as anything special either. Besides Garp, Luffy pre timeskip could fair against one AT FULL STRENGTH