So first of all, it needs to be said that "fun" is a different measure for everyone. This game so far strives to be fun for a wide variety of audiences, and Oceans in no way should take away from that.....it should only add possibilities and interest. My perspective comes from playing a lot of TFC and TFC+ mods for minecraft, and I think those guys nailed the size and scope of the ocean pretty well.
Overall, what I would like to see are oceans that are significant barriers for travel, but are not too daunting. Even a new character should be able cross them with a few in game days of preparation (perhaps to flee a bad seed rather than restarting the world). Ideal size to me would be somewhere between 20-40 RL minutes to cross, and perhaps some wind/sailing mechanics or currents could be added to spice up the travel. Ideally, it shouldn't be too boring to cross, but should be epic enough to feel like an adventure and require some preparation. Building a bridge should be out of the question.
With large oceans, you have the possibility for islands/archipelagos of various sizes, and you have new terrains you can introduce (salt marshes, river deltas, sand/gravel beaches, imposing fjords, volcanic islands, etc.) This will only serve to enhance the beauty and variety of the game as it stands now. Oceans also make great natural borders for different rock strata, different foliage, crops and wildlife, etc so there will be a strong incentive to explore. The ocean itself could play host to a number of different and interesting things to explore and collect for survival. Plus, the addition of salt water can allow for salt production without finding Halite. Salt water could even provide an initial product for the beginning distillation setup.
I love the idea of underwater ruins, boat wreckage, and sea creatures! anything to create more variety in our explorations!
Ocean travel shouldn't be gated behind metal, instead maybe have several tiers of boats that improve (T1 boats could be logs and maybe pitch, T2 could use planks and bronze and linen but be faster and maybe have item storage, etc). perhaps the more advanced ships would need to be constructed on a dry dock near the ocean and pushed in.