A car is absolutely a must. Although transportation services (taxis, UBER, etc.) are available, utilizing transportation services may prove extremely costly in comparison with renting a car.
Nonetheless, if you choose not to rent a car, there is a flat-rate fare from the San Juan International Airport to Loiza. This fee varies between $60-80. There can also be small surcharges for extra luggage or nighttime service, tolls, etc. Tipping is at your discretion, but normally about 10%. Alternate transportation services could be arranged in advance.
On Route 187 there are several restaurants, beach bars, pharmacies and gas stations with mini-marts. There are several grocery stores near Loiza on Route 3 (10 to 15 minute drive). An outlet mall, many fast food restaurants and family eateries are also available on Route 3 (Rio Grande and Canovanas). A Walmart Supercenter (open 24/7) is also located at the intersection of Route 3 and Rt 188 (Canovanas).