Goldeen can be found with the Good Rod at a moderate encounter in the following areas in Diamond and Pearl: Routes 203, 204, 209, 212 (North), and 214, Twinleaf Town, Lake Verity, Lake Acuity, and Lake Valor. In Platinum, Goldeen can also be caught in the Sendoff Spring, Route 208, and Celestic Town, in addition to the aforementioned areas.
In all three games, Seaking is found where Goldeen can be found, but using the Super Rod instead.
The lowest-leveled Goldeen will start out with; Peck, Tail Whip, Water Sport, and Supersonic. Peck is neat as it allows Goldeen to hit Grass, Bug, and Fighting for super-effective damage. However, it will become severely outclassed as time goes on. Tail Whip helps Goldeen score more damage, and is decent as an early move. Water Sport is semi-useless, as Fire is uncommon unless Goldeen plans to face the Rival's Chimchar - and let's be honest: it could fight it without Water Sport, too. Supersonic is not a good move at all, especially considering Water Pulse later on. At level 11, it gets Horn Attack. This is a much better move than Peck without super-effective damage, and should be kept unless Return is used on it. At level 17, Goldeen gets its first STAB, Water Pulse. Water Pulse is very good early on, and even has a chance of confusion to work with. Flail is at level 21, but it is more of a switch-or-die situation if Goldeen is at low health. At level 27, Aqua Ring helps its longevity a bit by healing a little bit every turn. Fury Attack at level 31 is generally mediocre compared to Horn Attack. At this point, Goldeen is likely to have evolved into a Seaking. Seaking gets Waterfall at level 40 - it is the holy grail for the family line: an actual physical Water STAB. Keep it, given the trainer will not receive the HM until after the 8th badge. Horn Drill at level 47 is a cheap move, that only works the best if Seaking is overleveled. Agility at level 56 is a good alternative to gain speed. Megahorn at level 63 is a stronger variant to Peck, despite the accuracy drop. As Seaking, it can relearn Poison Jab. It is an extraordinary move for Seaking to have, and it alongside Waterfall hit most Pokemon for at least neutral coverage.
Seaking, as a Water-type, can learn a flurry of Special Attack TMs, however they will most likely be null, as Seaking does not have a usable Special Attack. Blizzard and Surf, are the most notable, as the former is strong and the latter is STAB. Return is a strong physical move that Seaking can use as filler. Otherwise, Secret Power is about as good. Rain Dance helps set up rain for Swift Swim variants to make the most of, and a Damp Rock is the recommended item for this set. Natural Gift has a strange niche and works almost like a physical Hidden Power.
Regarding Platinum Move Tutors, the only ones before the post-game have these in store; Knock Off, Icy Wind, Snore, Swift, Dive, and Fury Cutter. None of these do better than what Seaking can normally learn in DP.
Grass-types: Grass Pokemon resist all of Seaking's weaknesses, and have Fire resisted by it in return. Grass Pokemon that are generally Special Attackers, are ideal to work alongside a Physical Seaking.