Must a man go to the Mikve?
The Gemara (Bava Kama 82a) records ten enactments that Ezra instituted and one of them was that a man who had a seminal emission had to immerse in a Mikve before praying and learning. The rationale for this was to prevent men, especially Torah scholars, from overly engaging in relations with their wives and it acted as a deterrent. In future generations, this enactment was rescinded since it was proved to be too difficult to abide by. Therefore, the Shulhan Aruch (O.H. 88:1) rules that if one experienced a seminal emission, whether accidental or deliberate, one may learn Torah or pray without first going to the Mikve.
Nevertheless, the Elya Rabba writes that there is a custom to be stringent and to go to the Mikve if one had a seminal emission. If one is not able to go to the Mikve, then one can have nine Kav (roughly 12 liters) of water poured on one’s body by someone else. Nowadays, if one wishes to be stringent but one cannot go to a Mikve, Rabbi Shmuel Wosner (Vol I:22) and Rabbi Ovada Yosef (Yabia Omer, vol. IV, § 88:3) say that a shower can act as having water poured on one’s body. Even though it is of Halachic importance to have water poured on to one’s body by someone else (Ko’ah Gavra), one may rely on the opinion of the Raavad that it is not essential for the pouring of the 9 Kav. In the same vein, a man would be able to immerse in a swimming pool, preferably when the filter is not on and while immersing under the filter hole . It should be emphasized that this only works for a man’s immersion, however a woman’s obligation to purify in a Mikve is a different category immersion altogether and a pool would not suffice.
If one cannot even shower but wishes to learn and pray in a enhanced state of purity, the Kaf KaHaim (K.H., O.H. 76:21), quoting the Emet LeYa’akov, says that based on Kabbalah, there is a special way to wash one’s hands forty times to achieve the same goal.
Interestingly, the HIDA (More Be’etzba, Tziporen Shamir, § 7) writes that there is a tradition that the negative force of the an accidental seminal emission is dissipated if one goes to the Mikve, confesses one’s sins and repents.
Summary: Strictly speaking one does not need to immerse in a Mikve following a seminal emission. If one wishes to be stringent, there are different methods of achieving a higher level of purity.