Why not? We have plenty of cables and so forth down their anyway and they are hardly that bad. Besides efficency losses for hvdc are at 3.5% for 1000km. So you could transport electricity from any place to any other place on earth with max loss of 49% using that technology. You also do not need millions of power lines and mostly not across oceans, but connecting islands with the next continent. Even then most islands with high enough population for nuclear to be even be reasonable are both large themself or close to some other continent or large islands. As for costs there is a serious private venture to built a direct undersea power line from Morroco to the UK. It is not cheap, but it certainly is not crazy. Obviously we have an electricity grid in most places in the world already, with 87% of the worlds population having electricity and nearly all who have no access are in Africa.