A stand-alone Single-Point LIDAR might be used to remove the high-end phone requirement.  Should interface to a phone USB port with a simple UART/I2C-USB adapter.
  Search Amazon.com for LIDAR finds 'Lidar Range Finder Sensor Module TF-Luna, Single-Point Micro Ranging Module 0.2 to 8m Compatible with Pixhawk, Arduino and Rasppbarry Pi with UART / I2C Communication Interface' $25.
  And 'MakerFocus TFmini-s Micro Lidar Module 0.1-12M Lidar Range Finder Sensor Obstacle Avoidance Sensor Tiny Module 1000Hz Single Point UART I2C IO Compatible with Pixhawk Ar duino and Raspberry Pi' $39.  
  The 2nd one is waterproof.