From: Madrid, Spain
The USA and the USSR enter the war when their war readiness (WR) reaches a certain threshold, each turn that WR increases randomly by certain point and by some actions done by the player.
For example there is a chance (25% or 50%) US WR increases any time an uboat attacks and damages or sinks a Transport Fleet, it also increases when an Axis power attacks a neutral country, when Vichy France is created, when the japs capture a chinese region, if Spain joins Germany or Britain falls to the Krauts. If Japan attacks the USSR...
A big boost in WR is given to the USA when either Japan or Germany attacks the USA.
WR also affects the output production capacity, for example when the US reach 28 WR it triggers the 2x multiplier for factories.
Each turn the WR is increased randomly by certain values in an interval. The USA enters the war when its WR reaches 37, or earlier if attacked.
For the USSR once the WR reaches 34 it enters the war, or ealier if attacked.
The USSR WR increases by 8 each time an Axis power attacks a neutral country that is adajacent to a Soviet controlled region. (So beware if you attack Turkey, or Persia)
It also increases by one if the Soviets increment the garrison in E. Poland under certain circumstances, or if the Germans violate the Soviet-German partition agreement by attacking the Baltic States or E. Poland before Poland has fallen.
Soviet WR increases when Vichy France is created, if England falls and when attacked by the Axis.
So now that WR has been explained to answer your question, my opinion is that Sealion can be attempted but you have to plan it flawlesly or it will not succeed (I'm talking against a human opponent), coordinating it with an attack on N. Africa and onto the Middle East would help, always keep an eye on Soviet WR or you might find the red bear stabing at your back. The cons are that you allow the soviets to build lots of units and increase their R&D levels, anyway many possibilities are opened.
Oh and forget about builidng a Glorious Kriegsmarine (apart from submarines) or you're doomed, it takes too long to be built, much better concentrate on aircraft, CAGs (carrier air grupes) if you want to inflict heavy damaged to allied shipping.