I can go along with Medium, though 12ft long should really be Large unless it's quite skinny. I guess it is at only a couple of hundred pounds, though!
I guess sharks are a lot like dinosaurs, eh?
I agree that a 12-foot mako ought to be Large. The thing is, most mako sharks are considerably shorter than that. The largest makos on record are about that length (according to Wikipedia the records are 3.40m to 3.70m and 4.45m).
After
wasting investing some time on the internet (this
researchgate page was particularly good) it appears male makos are mature at ~200 centimetres and females at ~250-300 centimetres in "fork length" (nose-to tail fork), with their weight in kilograms best-fitting to the formula W = 0.00004 × (L)².⁵⁹ [That should be "raised to the power 2.59" but this forum's formatting isn't supportive of equations].
If that formula is accurate, it comes out to 36.45kg for a 2m mako shark and 104.18kg for a 3m mako (90 lbs and 229 lbs).
That is REALLY skinny though. I'm leaning towards using Wikipedia's 60-135 kg (132–298 lb) as the basis for the average body weight of a shortfinned mako.
However, exceptionally large mako sharks start getting a lot stockier in build than smaller ones, which means the formula won't be reliable for the biggest specimens - a 12 foot mako seems to be about 800 pounds give-or-take with record catches of 570 kg (1,260 lb) and 600 kg (1,300 lb). There's claims for 770 kg (1,700 lb) and 1,000 kg (2,200 lb) makos 13+ feet long, but those are anecdotal or estimates.
Anyhow, to cut to the quick. What I suggest doing is quote the 12 foot length for an "advanced to Large" mako shark and use more realistic figures for a basic Medium sized specimen:
Like so:
Medium Animal (Aquatic)
Advancement: 5-6 HD (Medium); 7-9 HD (Large)
An average mako shark is from 7 to 10 feet long and weighs 125 and 300 pounds. Females are generally bigger than males. Exceptionally big makos can grow to Large size animals about 12 feet long. Large mako sharks are almost always female and are more stockily built than smaller specimens, typically weighing 600 to 800 pounds. Extraordinarily large makos can approach 15 feet in length and might weigh up to 1,300 pounds or possibly even more.
Let's keep the bite at 2d6+Str just to make it a tad better. Then Str 15 doesn't seem so bad. Dex 16 and Swim 70 ft sound ok. Keep the rest of the ability scores the same as the SRD shark?
I'm leaning more in favour of making the bite 1d8, which is still one step larger than the SRD Medium shark.
The Large mako proposed above will still have a 2d6 bite, although it will also have more HD than the BECMI original.