( EDIT: Edit #1 for descriptions )
I'll clarify for the HP Water situation.
Techno Blast water is stronger, but it requires you to fill up your item slot for just a little bit more base power in your water move.
Putting something like Life Orb in that slot gives not only HP Water higher damage, but also supports Genesect's entire moveset as a sweeper.
[email protected] orb
EVs: 252 Sp atk / 252 Spd 4 / HP
Life orb on this set is because of my personal preference of having Genesect be able to access bolt-beam without switching.
HP Water gets the rain boost on power, Thunder gets a nice accuracy boost in rain.
Bug Buzz is STAB. Ice Beam gets sweet coverage with Thunder. As trachy stated under Mike's answer, Flamethrower is also a nice choice there.