+4 natural is ok, and the acid write-up is good. Ready for the adult?
I'd prefer +5 but I'm OK with +4. I'll leave it up to Shade.
Let's get started on the adult then!
I mentioned earlier it has the same NA as a downsized Giant Stag Beetle (hereafter a GSB), so how about using that as a basis - it's about the right size (5-6 feet is half a GSB's 10 foot length and 4HD vs the GSB's 7HD).
Downsizing a GSB to Medium would give it Str 15, Dex 12, Con 13. I don't think it should have a higher Dex and lower Con than the larva (if anything, it should be the other way around), so I'll swap 2 points from one to t'other to make it Str 15, Dex 10, Con 15.
The adult doesn't have gills so should lose the Aquatic subtype, but I would think it should have the Hold Breath SQ since they can stay submerged for a long time. Somewhere between 4 and 8 times Constitution?
We'll also need to plug in in a 1d4 bite and a fly and swim speeds to match the AD&D adult diving beetle (also the same as a Giant Bombardier Beetle).
Altogether that would give something like:
Adult Diving Beetle, Giant
Medium Vermin
Hit Dice: 4d8+8 (26)
Initiative: +0
Speed: 20 ft. (4 squares), swim 40 ft., fly 20 ft. (average)
Armor Class: 18 (natural +8), touch 10, flat-footed 18
BAB/Grapple: +3/+5
Attack: Bite +5 melee (1d4+2)
Special Attack: None ?
Special Qualities: Darkvision 60 ft., hold breath [
Con*6?], vermin traits
Saves: Fort +6, Ref +1, Will +1
Abilities: Str 15, Dex 10, Con 15, Int 0, Wis 10, Cha 9
Hmm, matching the fly speed to the original's 9" looks too low to me - diving beetles are fairly good flyers, so I'd up it to 40 ft., or at least 30 ft.