What are the products of the dissociation of sodium bicarbonate in water? What is the relative pH of the solution?
It's not as intuitive as it seems and your questions are all legitimate questions, but sometimes even good arguments can't be used as evidence in chemistry.




  $\ce{NaHCO3 -&gt; Na+ + HCO3-}$, but that one doesn't involve water at all


Look at this reaction:
$$\ce{NaCl &lt;=&gt;Na+ +Cl-}$$

Even this reaction doesn't "involve" water in the schematics but is right, we assume that is a dissociation of a salt in water.




  $\ce{NaHCO3 + H2O -&gt; Na+ + OH- + H2CO3}$, but doesn't $\ce{H2CO3}$ decompose into $\ce{H2O + CO2}$?


Yes, but in fact this is an equilibrium:
$$\ce{H2CO3 &lt;=&gt; H2O + CO2}$$

And then you have to take into account another important equilibrium many time ignored:
$$\ce{CO2 (aq) &lt;=&gt; CO2 (g)}$$
So the amount of $\ce{CO2}$ present in the solution depends on by the type of system and the pressure exerted. Like a Coca-Cola bottle: if the system is closed, the $\ce{CO2}$ remains dissolved otherwise the equilibrium shift to the right and the $\ce{CO2}$ "escapes" until the rate between the $\ce{CO2}$ molecule dissolved and the $\ce{CO2}$ molecule escaped forms the solution is the same. 




  $\ce{NaHCO3 + H2O -&gt; Na+ + OH- + H2O + CO2}$, but that has water on both sides of the equation.


Okay, it is not a net ionic equation but the stoichiometry is still right. Maybe water is used to indicate that we are talking about a reaction that takes place in water.



Which reaction is not right?

All the reactions here are right, I think maybe they put some formal inaccuracies to help you, but in fact they are all part of series of equilibria that normally occur and can be described as:
$$\begin{multline}
\ce{NaHCO3 + H2O &lt;=&gt;[A] Na+ + HCO3- + H2O &lt;=&gt;[B] Na+ + H2CO3 + OH-} \\
\ce{ &lt;=&gt;[C] Na+ + OH- + H2O + (CO2(aq)&lt;=&gt;[C_2]CO2 ^ (g))}\end{multline}$$

The question should be more specific asking for the ultimate products of the reaction because all these are products of the dissociation of sodium hydrogen carbonate and because are equilibria is probable that all the species are present in different amounts when the reaction reach the equilibrium. However, the ultimate products are described by the third answer. And this equation can give you a hint about the acidity of the solution. When $\ce{CO2}$ escape from the solution (equilibrium $\mathrm{C_2}$ goes to the right) the concentration of the manly acid species goes down but the hydroxides from the equilibrium $\mathrm{B}$ are still present so at the end the solution in an open system with an atmosphere with a low partial pressure of $\ce{CO2}$ will have a higher pH, will become mildly alkaline.