Yes a relief force could enter the space via naval transport, although it would probably be cheaper to just try and break the opponents LOC, by fighting a naval battle.
The original poster asked about reinforcements which implies boosting the garrison under siege.
What you're suggesting, while true, results in a field battle.
A field battle after naval move could be disastrous for the attacker. Steven is right that, if you can get naval units in the adjacent sea zone, the assault cannot occur anyway.