300: Battle of Artemisia

Queen Gorgo of Sparta tells her men about the Battle of Marathon, in which King Darius of Persia was killed by General Themistocles of Athens ten years earlier. Darius' son, Xerxes, witnesses his father's death, and is advised to not continue the war, since only "the gods can defeat the Greeks". Darius' naval commander, Artemisia, claims that Darius' last words were in fact a challenge and sends Xerxes on a journey through the desert. Xerxes finally reaches a cave and bathes in an otherworldly liquid, emerging as the 8-feet tall "god-King". He returns to Persia and declares war on Greece to avenge his father.
As Xerxes's forces advance towards Thermopylae, Themistocles meets with the council and convinces them to provide him with a fleet to engage the Persians at the sea. Themistocles (the Athenian general) then travels to Sparta to ask King Leonidas for help, but is informed by Dilios that Leonidas is consulting the Oracle, and Gorgo is reluctant to side with Athens. Themistocles later reunites with his old friend Scyllas, who infiltrated the Persian troops and learned Artemisia was born Greek, but defected to Persia as her family was raped and murdered by Greek hoplites and she was taken as a sex slave, and subsequently left for dead in the streets. She was rescued and adopted by a Persian emissary. Her lust for vengeance gained the attention of King Darius and he made her a naval commander after she killed many of his enemies. Themistocles also learns that Leonidas has marched to fight the Persians with only 300 men.
Themistocles leads his fleet of fifty warships and several thousand men, which include Scyllas, Scyllas' son Calisto and Themistocles' right-hand man Aeskylos to the Aegean Sea, starting the Battle of Artemisium. They ram their ships into the Persian ships, charge them, slaughtering several soldiers before retreating from the sinking Persian ships. The following day, the Greeks feign a retreat and lead a group of Persian ships into a crevice, where they become stuck. The Greeks charge the Persian ships from the cliffs above and kill more Persians. Impressed with Themistocles' skills, Artemisia brings him onto her ship where she has sex with him in an attempt to convince him to join the Persians as her second-in-command. He refuses, causing her to push him aside and swear revenge on him.
The Persians spill tar into the sea and send suicide bombers to swim to and board the Greek ships with their flame bombs. Artemisia and her men fire flaming arrows and torches to ignite the tar, but an Athenian manages to kill one of the Persians, who falls into the tar carrying a torch, causing ships from both sides to explode. Themistocles is thrown into the sea by an explosion and nearly drowns before being rescued by Aeskylos, and stands by Scyllas' side as he succumbs to his injuries. Believing Themistocles to be dead, Artemisia and her forces withdraw. After recovering from his injuries, Themistocles learns that only a few hundred of his warriors and six of his ships survived the disastrous attack executed by Artemisia.
Daxos, an Arcadian general, tells Themistocles that Leonidas and his 300 men have been killed after Ephialtes betrays the Greeks to Xerxes. Themistocles returns to Athens and confronts Ephialtes. The deformed Spartan traitor reveals that Xerxes plans to attack and burn Athens to the ground. Ephialites is regretful of his actions, and is welcoming death. Themistocles spares him instead, so he can warn Xerxes that the Greek forces are gathering at Salamis. He then visits Gorgo in Sparta while she is mourning Leonidas' death to ask for her help, but she is too overcome with grief. Before leaving, Themistocles returns Leonidas' sword, which had been delivered to him by Ephialtes under Xerxes's orders, and urges Gorgo to avenge Leonidas.
In Athens, Xerxes' army is laying waste when Ephialtes arrives to deliver Themistocles' message. Upon learning he is alive, Artemisia leaves to ready her entire navy for battle. Xerxes suggests a more cautious plan but she still leaves for battle, ignoring Xerxes' advice. The remaining Greek ships charge into the Persians ships, and the two armies battle, beginning the decisive Battle of Salamis. Themistocles and Artemisia fight, which ends in a stalemate with both receiving severe injuries.
At this moment Gorgo, who had been narrating the tale to the Spartans, arrives at the battle along with ships from numerous Greek city states including Delphi, Thebes, Olympia, Arcadia, and Sparta, all of them uniting against the surrounded Persians. Daxos leads the Arcadian army while Themistocles urges Artemisia to surrender. Xerxes, watching the battle from a cliff, turns his back on her, acknowledging his invasion has ended in defeat. Artemisia tries to kill Themistocles one last time but is killed as he stabs her through the stomach. Themistocles and Gorgo take a moment to silently acknowledge one another's alliance as the remaining Persians charge while Dilios leads the assault. The three then charge at the opposing Persians with the whole Greek army.

Question: What do the Persias spill in the sea in order to burn the Greek ships?
Answer: tar