This actually brings to mind a larger-in-scope question I have been meaning to ask: What is the logical progression of the Mariculture Mod? I got into it almost purely for the fishing.... because I wanted a Koi pond.
Now though, I realize that enchanted jewelry is the real endgame for Mariculture.
How does one get from point A to point B?
If you're looking at enchanted jewelry as the endgame, then you're looking to craft the Celestial Mirror. This mirror carries a maximum enchantment level of 60, and is the only way to get some of the more powerful enchantments unique to Mariculture (Restoration, Reaper, Time Lord, Superman, etc).
The recipe requires 2 magic droplets, which can only be obtained by raising clownfish.
It also requires 2 nether stars, which can be crafted from wither skeleton skulls.
So the endgame is approached from 2 different angles, fishbreeding and good old mob hunting. Keep that in mind as you're progressing. Don't neglect armor and weapons. Put something aside to make yourself hard to kill when you enter the nether.
The regular magic mirror has a max enchantment level of 30 and can enchant jewelry that can help you survive the nether:
Fall Resistance (10 blocks per level)
Inferno III (immunity to lava and fire damage)
Poison Ivy III (immunity to poison)
As far as overworld, You're trying to gather as much coal, clay, sand, aluminum and limestone (chisel) as possible. Dyes to be on the lookout for (black, yellow, brown, light blue).
Try and shoot right for the scuba gear as soon as possible. Having that turns the ocean floor into a supermarket of ores and other valuable material.
Use the reed and wooden fishing rods until you have enough titanium to make an RF fishing rod. It costs 2 titanium rods and a titanium battery, but you'll never need another rod as it never breaks. Only needs to be recharged. Don't ever make a titanium rod to fish with, it's a waste of titanium.
The two resources you are going to need the most of is natural gas and quicklime.
I use a snorkel to scout the shallow coastline around land masses, looking for a gas leak on the sea floor. I carry an air pump, a manual turbine to power it with, and plenty of Thermal Expansion portable tanks. I carry between 6-8 of the 32 bucket tanks because that is usually sufficient to completely harvest a gas pocket in one shot.
For the scuba gear you'll need 34,000 mb of gas for 17 units of neoprene, 150,000 mb of gas for 5 units of hard plastic (4 for the flippers, 1 to soak in molten glass to make the transparent plastic for the mask lenses).
When you have the scuba gear, resources are not going to be an issue going forward. You're going to have access to mineral deposits on the sea floor that many players simply won't be able to reach.
Best to stop there with the advice. Maybe you have some follow up questions to some of this?