When “Umi” is on the field, and “Amphibious Bugroth MK-3” declares a direct attack, and “Umi” is destroyed during the Battle Step, “Amphibious Bugroth MK-3” can no longer attack directly, so a replay occurs.
Netrep Rulings:
The effect of “Amphibious Bugroth MK-3” may be used regardless of which player controls “Umi.”
If “Umi” is destroyed in response to “Amphibious Bugroth MK-3” declaring a direct attack through its effect, a Replay is triggered.
If “Umi” is face-up, but negated, “Amphibious Bugroth MK-3” may still attack directly.
If “Umi” is destroyed or removed from the field as a response to the declaration of an attack by “Amphibious Bugroth MK-3”, a replay is triggered.