There are several main issues with Manaphy; the first of which is the lack of switch cards, meaning it's very difficult to get it active without effort, the second of which is that it's very easy to play around, and the third of which is there's no room. Blastoise is extremely tight space as is and can't afford to add a card that may barely even help it.
On the topic of other techs, I think they're a waste. Once Blastoise sets up, it does such an absurd amount of damage in a short time that it doesn't need anything else to help it; things like Suicine and Silver Mirror are pointless since this deck is designed to be offensive and things like Kyurem PF are useless because Blastoise serves as a fine attacker against those, not to mention they aren't really that popular at the moment.
On the topic of other techs, I think they're a waste. Once Blastoise sets up, it does such an absurd amount of damage in a short time that it doesn't need anything else to help it; things like Suicine and Silver Mirror are pointless since this deck is designed to be offensive and things like Kyurem PF are useless because Blastoise serves as a fine attacker against those, not to mention they aren't really that popular at the moment.