For every point of the entire party’s combined maximum energy capacity (aka their total burst costs), the wielder’s burst damage is increased by 0.12%. A maximum of 40% damage increase can be gained this way. Its condition is very easy to achieve and only gets better in teams with higher-cost bursts. Additionally, here’s a cheat sheet of how each Watatsumi Wavewalker synergizes with all current playable characters.
Great means the weapon works well with the character, and it’s one of their best 4-star weapon options. Decent means that it works on the character, but there are other far better options – usually other 4-star gacha weapons. Not recommended means that it doesn’t synergize well with the character – or there are significantly better options (usually other craftable weapons or 3-star weapons).