The Ukrainian fleet is harbored right next to the Russian fleet at Sevastopol-- all it would take would be a couple of Ukrainian cruisers firing point blank to cause massive damage.
Russia could undoubtedly 'win' in the end, but the cost would be massive, and would also probably involve loss of access to Western markets and cash.
The bigger danger to Ukraine are 'separatists' funded by Moscow and aided under the table with weapons or Russian special forces.
Russia wants a federal solution - or barring that, its fully prepared to aid and fund separatist forces - and can put in troops to protect the local population.
Kiev doesn’t have much choice and the country is teetering on the edge of a breakup.
Ukraine ground forces consist of 57,000 personnel, not all combat. They have 2 armor brigades which use the Ukrainian upgraded T-64 (686 total tanks) as their main battle tank. Most of the rest of their equipment is leftover from the Soviet era. No matter how well trained and dedicated their troops are, it's going to be a short battle.
Biggest and more real danger to Ukrainians is state their economy. They have no money to pay pensions, to support state activity. Ukrainians must be ready to marauding gangs for they destroyed police. Nobody sane enough will give money to this fault state.