How to protect a boat from spells?

[Temennigru]

(lvl 9)
On our last encounter, our caravel was almost destroyed by a sorcerer who could cast fireballs.
We tied the sorcerer up and started questioning him, but for some reason he was still able to cast and cast more fireballs on us.

Is there a way to prevent our boat at least from being hit by spells? Or preventing a single person from casting spells?

[ExLibrisMortis]

You can animate your boat and buff it like any creature, though that may get quite costly and/or time-consuming. Magically hardening the ship can help, as can adamantine-plating the hull.

There is also the Dragonlance prestige class, Sea Mage (Towers of High Sorcery page 33). It gives a fun ability:

Once per day, when the sea mage is preparing his spells, he may imbue a number of spells equal to his sea mage level into the boat itself. The maximum level of any spell to be imbued is one less than the highest level spell the sea mage can cast, and imbued spells use up spell slots like other prepared spells. The sea mage casts an imbued spell as normal (including requiring all verbal, somatic, and material components), but, so long as he is in contact with the boat at the time, he and the boat are considered to be one for the purposes of determining the spell’s range. Specifically, touch spells can be directed to any target in contact with the boat, and any point in or on the boat can serve as a spell’s point of origin.
You may be able to use this to cast personal-range spells on your caravel.

[Cerefel]

You could try using Bestow Curse to bring down the sorcerer's casting stat, which will almost certainly deny access to a lot of their spells

[marphod]

To prevent casting spells, have someone stand next to them with a readied action to hit them if they try to cast.

They are helpless, so ye old rogue gets SA damage. If you want the sorc to live, use a whip or sap.

Also, count slots used. A stilled, silent fireball is a 5th level spell, a level appropriate opponent only has so many. make sure they arn't weilding any metamagic wands.

[Telok]

Three solutions.
1: dedicated counterspelling
2: kill the enemy before they can cast spells
3: turn you boat into a creature

You can try spending money and time enchanting your boat but a single dispell will turn the boat back to normal long enough for it to be wrecked again.

[Andion Isurand]

There are some spells in Stormwrack, and I think a few other places, that set the precedent of targeting an entire ship (along with its crew and contents).

I know you're probably not looking looking for homebrew solutions, but one could always try brewing some custom spells (http://magerune.blogspot.com/2015/07/vessel-and-vehicle-spells.html), possibly based in part upon other spells, that might affect an entire vessel or vehicle to reinforce or secure it.

[Telonius]

Blindfold him and place him in a box. Drill a few small holes, large enough to breathe through but not large enough to cast through. The only one he'll be Fireballing is himself. Also, make sure his sleep is disrupted.

I'm assuming you need some information from him. Depending on your alignment: start lowering the box into the water. "Now look, friend. You have something we need to know. If you tell us, we let you go on the next island we come to. Beautiful place, lots of coconuts, nice weather, no volcanoes, luaus every Friday, girls with lots of coconuts. You could do a lot worse than end up there. You don't talk, and we'll keep lowering you down into the water and get an education in the Drowning rules. Our Cleric knows how to cast Speak with Dead, so we're getting the information one way or the other; it's up to you whether you're alive to give it."

(EDIT: Obviously have someone down in the water for when he tries to escape)