You can't make a nuke look like a power plant explosion; they're simply too different. No nuclear power station can explode in that fashion.
A bomb over the ocean wouldn't demonstrate any new capacity, and would be seen as the bluff it would almost certainly be.
A nuke on Ukranian soil would further open the floodgates of Western aid, expand sanctions, and push more nations firmly into the EU/NATO fold as Finland and Sweden already have been.
A nuke on Ukrainian soil also has the problem of the prevailing wind direction being from west to east. Detonating a nuke on Ukraine looks a lot like detonating a (smaller) dirty bomb on Russia.
A bomb over the ocean wouldn't demonstrate any new capacity, and would be seen as the bluff it would almost certainly be.
A nuke on Ukranian soil would further open the floodgates of Western aid, expand sanctions, and push more nations firmly into the EU/NATO fold as Finland and Sweden already have been.