Hey Leafy, Your Starmie is probably not particularly useful.
You have water gun for attacking, which is awesome, but one of the reasons Psychic isn't very useful to an attacking Starmie is the current popular defender list. While Psychic isn't a very fast attack, it's not a bad attack, but to make a Water Gun/Psychic Starmie useful we'd need a number of good gym defenders which would perform poorly against Psychic attacks.
After more pokemon are released you may see the desire for such a Starmie rise, but for that to effectively happen you'll need a release of strong defensive poison and fighting types. For Poison, you'll be looking for Amoongus (gen 5), Skuntank (gen 4) and Dragalge (gen 6), and for Fighting you'll be looking for Gallade (gen 4) and Hariyama (gen 3) so it's unlikely that you'll see any use for such a Starmie for quite some time.
Right now that Starmie will match up best against Nidoking and Nidoqueen, neither of which are currently considered top-tier defenders.
When looking at your Starmie as a potential defender, Psychic becomes a much more attractive option, but now it's Water Gun that's lacking. The bright side of your Starmie is that it will cover against both Arcanine and Machamp both of whom are decent threats. The Downside is that Starmie as a pokemon makes for a poor defender because it doesn't have enough health or defenses to last long enough to get off Psychic as often as would be necessary to make it worth it. You can have a Defensive Starmie for sure, but I certainly wouldn't pump any precious dust into it.
For now I recommend saving up your star dust and trying to cover the two extremes (attacking, and defending) with pokemon specialized for those roles, rather than finding a good middle-ground. Right now the mechanics (a gym-bound pokemon stays there and cant help you assault) don't support much overlap between the two roles on an individual pokemon.
Good luck, and catch em all!
