Eluge, the Shoreless Sea

Eluge's power and toughness are each equal to the number of Islands you control.
Whenever Eluge enters or attacks, put a flood counter on target land. It's an Island in addition to its other types for as long as it has a flood counter on it.
The first instant or sorcery spell you cast each turn costs {U} (or {1}) less to cast for each land you control with a flood counter on it.

Bloomburrow

Bloomburrow Promos

A land affected by Eluge’s second ability stays an Island until the flood counter is removed, even if Eluge leaves the battlefield.

If the first instant or sorcery spell you cast in a turn doesn’t cost any {U}, its cost will be reduced by {1} for each land you control with a flood counter on it.

In the case where the first instant or sorcery spell you cast in a turn does cost some {U} but you control more lands with flood counters on them than the number of {U} in that spell’s mana cost, all of the {U} in the cost are reduced first before moving on to the generic mana. For example, if you control four lands with flood counters on them and you cast Season of Weaving (a sorcery with mana cost {4}{U}{U}) as the first instant or sorcery spell of your turn, its cost will be reduced by {2}{U}{U}, meaning its total cost will be {2}.

The ability that defines Eluge’s power and toughness functions in all zones, not just the battlefield.

The land retains any land types and abilities it already had. An Island has the ability “{T}: Add {U}.”

To determine the total cost of a spell, start with the mana cost or alternative cost you’re paying (such as a flashback cost), add any cost increases (such as kicker costs), then apply any cost reductions (such as that of this ability). The mana value of the spell is determined by only its mana cost, no matter what the total cost to cast that spell was.