I believe that at that point in time, Route 222 is a better spot to train.
There is one fishermen named Alec on that road who has two Gyarados (the first Gyarados might be a Magikarp for the first few rematches) and you have two more trainers around nearby which means you can use the VS Seeker a few times and still get quite some rematches. Those should give you some EXP.
You also are on land (instead of being on the sea) where you can use your bike to charge up your VS Seeker faster than on Route 223.
If you manage to get a Lucky Egg (obtained from Chansey typically on Routes 209, 210 and in Trophy Garden), you'd be able to speed this up even further.
Once your Pokémon are in the vicinity of level 55, you should be able to take on the Elite 4 with some items to help; Hyper Potions, Revives are the basic ones you can use.
Unfortunately in Pearl, there are not as much trainers who can do rematches, when compared to Platinum.
Victory Road can be another good spot, if you don't feel like waiting for the VS Seeker (or don't want to rely on its randomness when trainers don't want to rematch). Having a Pokémon with Illuminate (e.g. Chinchou) at the first slot in your team, you can get plenty of wild Pokémon encounters there and in a few minutes, you should be ready to take on the Elite 4.