I need a ruling!

Dear Dragon Magazine: 

One of my players was in a boat and wanted to attempt to backstab a shark that was attacking the boat. 

I posed this question to the google hangout old school grognard chat that I'm a part of.  For the sake of argument I was playing basic fantasy.  Here my friends are the wonderful answer's that I received.  To keep with the anonymity of the chat group, I've just labelled everyone who took part in the conversation with numbers. 

1:  Does the shark have a backside? 

Technically the sneak attack is because the target is unaware of the attack about to hit.  I don't see why the thief would not be allowed to sneak attack target who was not aware.  

2:  Backstab is only with discernible "back" so shark is eliminated altogether. 

Grognard's quoting different editions....

2:  old school technically "unnoticed"  but considering a sharks eyes depending on species are positioned in such a way as to see above their back, the point is moot.  It's called  BACK stab, so it is clearly meant for humanoids only! 

1:  sharks have a spine, and therefore a back. 

3:  and you have to account for the fish's lateral line being able to detect living creatures in the vicinity.

2:  a spine not a back so WRONG! take a biology class again.

3:  the shark is always hunting, always hungry killing machine.  your pathetic sneaky monkey skills have no power over 400 million years of predation perfection! 

1:  I was going to say that a shark's eyeball positioning actually allow it to see nearly 360 degrees around its body and would therefore almost be impossible to attack it unawares. 

HERE'S THE MOST IMPORTANT PART 

1: So while the rules would probably allow the attack a common sense approach to biology would probably prevent the attack  However any good DM worth his salt would probably make a judgement call in this situation and allow the sneak attack to happen for the sake of the story and fun of the game. 

Here's the official rule from Basic Fantasy SRD.

Finally, Thieves can perform a Sneak Attack any time they are behind an opponent in melee and it is reasonably likely the opponent doesn't know the Thief is there. The GM may require a Move Silently or Hide roll to determine this. The Sneak Attack is made with a +4 attack bonus and does double damage if it is successful. A Thief usually can't make a Sneak Attack on the same opponent twice in any given combat.

The Sneak Attack can be performed with any melee (but not missile) weapon, or may be performed bare-handed (in which case subduing damage is done; see the Encounter section for details). Also, the Sneak Attack can be performed with the “flat of the blade;” the bonuses and penalties cancel out, so the attack has a +0 attack bonus and does normal damage; the damage done in this case is subduing damage.

The one thing that I didn't realize was that a sneak attack only applies to Melee weapons.  In the case of the shark the player was attempting to use his bow.  I've decided that I'm fine with missile weapons as well.  In my game I will allow a sneak attack as long as the player can remain unseen, or the monster is unaware.  the only weapons not useable for a sneak attack would be two handed weapons. 

What I ended up doing was allowing the attack because when I described it the sharks were only aware of the boat and were running into it trying to flip it over.  I allowed it for one round.  There were 2 sharks and after 1 died I made the other shark make a morale check (which it lost).  It decided to swim away.  I mentioned it to the Grognard chat, which brought up a whole lotta "sharks shouldn't make morale checks! they are sharks, they are hungry, they should have a morale of 12!". 

In BF the bull shark has a morale of 7. 

What would you have done?