Pielorinho said:
I mentioned one "philosophical" reason above for allowing it, and now here's another:
One of the golden rules of DMing, one I sort of picked up from Piratecat, is this: Let the PCs' abilities work. Making the game tougher by saying, "No, you can't do that," is just frustrating for players. When a player says, "Can I do X?" your default response should be yes.
That doesn't mean the game is a cakewalk: you give the same consideration to the bad guys, too. It just gives PCs a chance to shine, a chance to feel useful, and makes the players happy.
Daniel
I'm diverging off of the main topic and I hope Quasqueton will forgive me.
I picked up the same philosophy from the same source. And I agree with your earlier assertion that if the whole "swimming through the tunnel" scene is not really a challenge to the party but really just more of a exercise in number crunching (i.e. if the Handle Animal check amounts to a "Take 20") then it is best to hand wave it and get on with the fun. But sometimes the seemingly mundane task is made a true challenge for the party due to limited resources or extenuating circumstances.
For example, in my current campaign, the party Wizard gained the Teleport spell. Now the party can go anywhere they want. But there's a problem: The weight limit of the spell means that they can't move the whole party at once and it would take a minimum of two trips to get from A to B. But my resourceful players found a way around this: The Druid casts Feathers on the party members, turning them into relatively small birds and THEN the Wizard Teleports them. Simple solution but one that I didn't "hand wave". It was, in effect, a challenge that the party overcame.
Another example from much earlier in the same campaign: The party has to get across a wide river. They have a boat. But it is only a dugout canoe and can only hold two people and their gear. Still it is just an exercise in patience and taking 10 on a few rolls to successfully maneuver the boat back and forth across the river a few times. Normally, I'd hand wave this. But the party is being tracked by a band of Gnolls who are fairly close behind them. So they puzzled out a way to get them all across as quickly as possible and managed to escape just ahead of the Gnolls who were left to throw a couple javelins at the final departure of the canoe while the PC's made rude gestures at them. I didn't intend the river as a challenge but it became one due to other circumstances.
Final example: I ran a home made version of In the Dungeons of the Slave Lords at the first NC Game Day. The party had to cross a gap in a stone bridge across a chasm. The gap was 10 feet across. Normally no big deal for the average 5th level party and I'd hand wave it (or they'd suggest an easy solution so quickly that I wouldn't need to hand wave it). But this party is all but naked. Their lack of supplies means that this simple gap in the bridge is supposed to be a challenge for them. So using various means, the party manages to get across the gap and (hopefully) that process if fun all by itself.
In a feeble effort to relate this to the main topic of the thread, I'd say that the "underwater tunnel" bit sounds like a good challenge to me IF it would be difficult for most of the party to navigate it. If the NPC Animal Companion is the only difficulty, I'd play fast and loose with the rules to get the critter through the tunnel and proceed with the main part of the game.
The one caveat I'd offer there is that if the players in question love figuring out these sorts of challenges (i.e. they're Tacticians like me), I'd let them hash it out so long as it didn't threaten to absorb the majority of the session or something like that.