Splash Plate, Sea Incense, Wave Incense, which is best?

I currently have 3 items—splash plate, wave incense, and sea incense—which all apparently increase the power of water-type moves.

I only plan on having one water-type pokémon in my party at a time, and I don't care about breeding at all. So, based on power boost alone, which of these is the best to use? Is there any general rule for ranking power boosts of these sorts of items?

In case it matters, my water-type moves are Surf, Waterfall, and Brine.

All thee in Gen 4 (also, Mystic Water) boost the power of water type moves by 20%, so they are all functionally the same in battle.

The only difference between them in battle (for most Pokemon) is that is that if you Fling the Splash Plate, Fling will have a base power of 90 (Mystic Water = 30, Incenses = 10). Arceus's type changes to water when the plate is held, and changes the type of its signature move, Judgement, to water.