In hydration of an alkene by oxymercuration-demercuration, for an asymetrical mercunium ion, the more substituted carbon atom is attacked by water.
Why is this the case? Since R groups are electron donating, it should make the more substituted carbon less electrophilic, also water would like to attack the carbon having less steric hindrance but the reverse is true.
Using 1-methyl-1-cyclohexene as an example, the mercurinium ion is not symmetrical. If it were, the positive charge would all be on mercury. The ion is unsymmetrical with part of the positive charge on the more substituted carbon. Why this carbon? Follow Markovnikov's Rule: tertiary carbocations are more stable than secondary carbocations. This bias is manifested in the transition state permitting water to attack at the more substituted carbon. The addition is anti. I chose 1-methyl-1-cyclohexene as an example because adding a tertiary butyl group to the 4-position produces more subtle aspects of the reaction mechanism.
While structures 1 and 2 above are both tertiary alcohols, only alcohol 1 is formed. This selectivity is controlled not only by the anti-addition but also by the more favorable chair-like transition states (red arrows) over the boat-like transition states (blue arrows). These issues are provided in greater detail at http://ursula.chem.yale.edu/~chem220/chem220js/StudyAids.html#Mechanisms as PowerPoint and pdf files.
Oxymercuration is very regioselective and is a textbook example of reaction obeying Markovnikov's reaction. This phenomenon is explained by examining the three resonance structures of the mercurionium ion formed at the end of the first step during reaction. In one stage you will have a tertiary carbocation, which of course is a very reactive electrophile. The nucleophile will then attack the mercurinium ion. Therefore, the nucleophile attacks the more substituted carbon because it retains a more positive character than the lesser substituted carbon.
And about steric hinderance: oxymercuration is an anti-addition.
