Blastoise vs Dragonite

better than no ice move

Still probably lose to dragonite

Cp?? It can kill dragonite's got one that kills 2.4k - 3k dragonites..

Water attacks being 'not very effective' against Dragonite is the problem here, even with the Ice Beam. Wouldn't be my first choice against Dragonite, it really depends on what other Pokémon you have and what CP it has compared to the defending Dragonite.

It is more effective than having a Blastoise with no ice beam. I'd rather use different pokemon such as dwegong or a wigglytuf against a Dragonite.

If your question is whether to power up your current Blastoise I would say no. Especially not if you are considering it to use as main attacker against Dragonite as there are better alternatives in my opinion.

Lapras with frost breath blizzard is the true dragonite killer .. Pointing out the obvious I know

I got a Blastoise with 84 IV, with Bite and Ice Beam, it's quite easy to take down a Dragonite with up to 2800 CP, and when the phone helps in dodging, I can get almost two Dragonites around 2300 CP... So i think it's very effective in a general way. (this blastoise is 1564 CP)

I have a Ice beam Blastoise, Goldduck and a Slowbro all around 1.8k-2k and use them to easily take down 2.5K+ Dragonites down all the time. Great thing about all these mon's is I evolved them or caught them in the wild and didnt have to use any stardust to power them up.

Since Ice moves have a 160% damage bonus on Dragon/flying they are quite effective even without STAB. Fairy mon's with STAB bonus will hit similarly hard as non-ice types with ice moves on Dragonite.

Undoubtedly a 0% IV 2100 CP Blatoise/Gold/Slow with Ice beam is not as good a 100%IV Dewdong maxed out at 2100 cp with frost breath and Ice beam/blizard. Ice mon's have the benefit of STAB and Dragonite is double weak to ice since its both flying and dragon so ice moves from ice mon's deal almost 200% damage to Dragonite .

Unfortunately a 2k CP Dewdong will cost you 200K + stardust so its really a question of long term game strategy or preference. I personally try to avoid using stardust on gym attacking Pokemon. Instead I am currently using as little stardust as I can to hold 12-15 gyms and am saving the rest for Gen 2 and upcoming events.