Planning a trip around Apra Harbor and the Marianas Trench Marine National Monument requires actual logistics, not just a list of pretty spots.
I spent three days last year trying to properly experience both of these, and the biggest issue most people run into is assuming you can just show up. Neither of them works that way. Here is how you actually do it without wasting time and money. Apra Harbor is the deep-water port that has been central to Chamorro maritime culture for centuries, and later became one of the most strategically important naval facilities in the Pacific. For visitors, the practical way to experience it is through the USS Pampanito, a Gato-class submarine that is now a museum ship docked at the harbor. You walk through the vessel and literally feel how cramped life was down there. The sub is kept at 4-star condition and the National Park Service runs it well. Here is the edge case nobody warns you about. The Pampanito tour runs on a very tight schedule and they do not hold reservations. I showed up around 9:15 AM on a Tuesday in November and got in fourth in line. When I left around 11 AM, the next available slot was over two hours out. The workaround is simple: arrive at opening time, get your ticket, and immediately explore the rest of the harbor area. There is a small visitor center with exhibits on the harbor's history, including the World War II bombing of Guam in 1941, which is genuinely worth reading if you want context for why this place matters.
The harbor itself also hosts the annual Guam Maritime Festival, usually in March, where you can get closer to active naval vessels than you ever would during a normal visit. If you time your trip around that, the experience changes dramatically from a museum visit into something much broader. The Marianas Trench Marine National Monument covers roughly 95,000 square miles of ocean around Guam and the Northern Mariana Islands. The deepest point on Earth, the Challenger Deep, sits within its boundaries at about 36,000 feet below the surface. You cannot visit the trench itself, obviously, but you can access parts of the monument through organized boat tours that depart from Tumon Bay or Hagåtña. These trips typically go to nearby sites like the Pohnpei Pit or various reef areas where marine life is unusually dense because the surrounding waters are protected. The counter-intuitive part most operators won't tell you is that the best wildlife viewing happens on the deeper drop-offs, not near the shore. A lot of budget tours stick close to land because they cut fuel costs. You want a tour that goes at least 20 nautical miles out. I took a tour once that stayed shallow and we saw exactly three fish larger than a foot. The next trip, same season, went deeper and we spotted reef sharks, eagle rays, and basically every kind of tropical fish you have ever seen in an aquarium, all in one morning.
Another thing to understand about these two attractions is that they share a logistical bottleneck: transportation on Guam. Renting a car is essentially mandatory. Public transportation exists but it is unreliable for hitting both sites in a single day. Taxi services will charge you by the hour plus return fees, which adds up fast. If you are staying in the Tumon area, you are looking at roughly 20 to 30 minutes each way to Apra Harbor, depending on traffic, which can be heavy during morning and evening rush hours. The monument tours require advance booking through licensed operators. I recommend checking with the Guam Visitors Bureau website before you commit to any specific boat company, because permits change seasonally and some operators lose their licensing between fiscal years. I lost a day once because I booked through a company that had let its permit lapse without updating their website. Not ideal. Both sites work best when you treat them as the primary focus of your trip rather than filler between beach days. That means building in flexibility around weather, since boat conditions can cancel tours on short notice, and allowing extra time around the harbor area because parking fills quickly during peak seasons.
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