Well good riddance to 'Undertow'. To me personally, this ability was tied with Inaros 'Devour'/'Sandstorm' for most boring ability in the game.
My only gripe with the rework is that Hydroid (still) seems to be very reliant on augment mods. Want your 'Corroding Barrage' to not be useless against other factions besides Grineer? Gotta use 'Viral Tempest'. Want to heal yourself and support allies? Gotta use 'Rousing Plunder'. Want to simply create more loot for yourself and allies? Thats a no-brainer, so still gotta use 'Pilfering Swarm'.
If Hydroid is supposed to be pirate-themed and the new 'Plunder' ability is supposed to reinforce that idea, then I personally think 'Pilfering Swarm' should have just been made part of the base ability. Also, as someone else pointed out, the new 'Plunder' ability doesn't work like Chroma's 'Vex Armor' where it refreshes the timer and keeps the buffs strength after recasting when it is still up. Instead it just completely resets, which might causes you to get a much lower value like seen in the stream. I hope this can be changed so 'Plunder' works more like Chroma's 'Vex Armor' when recasting.