I'm about to sound like a nerd but here goes.
First of all you need to remove mew, I'm assuming you're only using this team in-game, but in any comptetitive match that would classify you as an uber (legendary) team and you would get owned. A good swap would be Alakazam or Gengar, both are the supreme Psychic types. Replace Jolteon with something like Electrode and/or Magneton, both have better stats. Next, replace Blastoise with Feraligator, Feraligator has better attack and special attack stats compared to Blastois' defense and special defense, assuming you're semi-inexperienced you won't be utilizing much of Blastois' defense. Lapras can be replaced with something like Gyrados, but I honestly have no idea why you would opt to have to two water types on one team. A well trained ground and electric type could overpower your whole team. I would opt for a ground or dragon type such as Skarmory, Dragonite, or Tyranitar, all of which can effectively level even a well trained team. Arcanine and Charizard should be alright, but again, you're repeating types and both have exposures, if you do have to take one out remove Arcanine.