It also means faster warming, because of reduced albedo - and part of that warming is transferred to the adjacent land ice (esp. Greenland) and permafrost, melting it in turn, which translates to higher sea levels. For bonus points, when permafrost melts, it releases methane, which is also a greenhouse gas.

Then there's reduction of ocean salinity, affecting existing ecosystems established around the current level (and with a rate of change too fast for gradual adaptation).

And not to forget about the disruption of existing ocean currents, which significantly affect weather patterns. And if I remember correctly, atmospheric circulation would also be affected.

Open shipping lanes are well and good, but when you can't grow the usual crops where you used to be able to grow them, because the climate is no longer appropriate for them in that location, and seafood catches are lower because the ecosystem is disrupted in a big way, you may well find that all that extra cargo capacity is used up just shipping food to places that used to be able to grow their own.