Heat Blade is a weapon that only Dart can wield, which is first found at Kazas. It is imperative that it is found, and used exclusively until Fueno, because of the strength boost it provides over the Broad Sword and Bastard Sword. The fire element that accompanies every slash makes the Heat Blade deal 50% more damage to water enemies: 27 instead of 18. However, since there is no other weapon with as much base damage, its use is not optional until it is replaced, except against Lenus and Regole. It is more or less replaced by the Falchion from Fueno (26), and completely replaced by the Fairy Sword from Ulara (39).
It is too weak against the other seven elements in the Undersea Cavern, and Kashua Glacier where it can be bought, to be considered an advantage on its own. The Falchion is only one point less, but does that damage to non-Water enemies, which are in significant numbers in both locations, and the vast majority elsewhere.
However, in those locations it may not be a great handicap; the other characters can concentrate on the non-Water enemies. The Heat Blade will be an advantage if, for example, Dart can finish off the Water enemies in one hit instead of two, or two hits instead of three. If it is normally two, then it will only be a hindrance, taking enemies to the red, where they spam their spells.
Until Fueno it serves as a decent normal weapon, even against Fire enemies (before Fueno: Hell Hound, Knight of Sandora, Will-O-Wisp). The Heat Blade's bonus damage against water element enemies is hardlworks with the first fight against Lenus (there is no alternative in any case) but it should be replaced for the Undersea Cavern. If the player forgets to equip it for the fight against Lenus' Dragoon form and Regole, it would probably be no great loss. More important to remember to switch out of Dragoon form for the end of the battle.