Blastoise +80% 15 ATK ( Bite/Flash Cannon )

Impossible to answer without knowing its level & your level & what the current Gym Levels are in your area.

At level 30, based on IVs and my own blastoise's I would guesstimate that Blastoise would be about 2k CP.

If your Blastoise is close to level 30 AND if 2k puts you in middle 4-7 range of Gyms in your area, I'd say go for it.

((Scale the Above Downward if your not level 29 at above 69 CP per level. So if your level 27, subtract 138 and it would max around 1860. but the 2nd part wouldnt change would 1860 put you in 4-7 range))

Pokemon Go is often a Math Problem :>

Also .. what other defensive pokes do you have now and which others are you considering raising?

Personally I would plan out how you are going to get 10 defenders who are tier 1 to 1.5 on this site & can get 2400 CP eventually. That limits us to a really small # of pokes: Dragonite, Snorlax, Lapras, Exeggutor & high iv Vaporeons.

If you don't have any of those creatures and you have a ton of dust (300k+), then I suppose you don't have much to lose by raising the Blastoise for now. But surely you have some vaps anyway?

No. I would only power one up with ice beam, because that will be SE against the grass types people will generally attack it with, plus it will do neural damage to electric pokemon. Flash cannon is pretty lame and will be resisted by electric attackers and do neutral damage to grass attackers, as opposed to the SE from ice beam.

Neither flash cannon nor ice beam get STAB, but as a defender ice beam is a great move. Flash cannon isn't really good for anything.