TikZ word search diagram

Not an answer (too long for a comment and requires image uploads)

I've created a Python program to read a list of words from a text file and create all the LaTeX code necessary to produce the game and the solution, as shown below. If you are interested I can try to host it on GitHub.

With soup package:

\documentclass{article}
\usepackage[usetikz=true, highlight=true, highlightcolor=orange, linecolor=red]{soup}

\begin{document}
\begin{alphabetsoup}*[6][6][\sffamily]
\hideinsoup{2}{2}{downright}{a,i,r}
\end{alphabetsoup}

\end{document}

I find it too tedious to type a 15×15 matrix manually and then, when highlighting the various “words” placed horizontally, vertically, or diagonally, to count the exact cells where each word begins and ends. It is much easier to let LaTeX do this automatically.

The code below requires only entering the rows themselves; LaTeX will construct the table. The table supports both row and column numbering. Both the numbering and the grid can be turned off using the keys \WSsetgrid and \WSsetnumbers. In the first example below, both switches are in the off position.

\documentclass[tikz,border=5pt]{standalone}
\usepackage{xstring}
\usetikzlibrary{calc}

% ===== Switches for grid and numbering =====
\newif\ifWSshowgrid
\newif\ifWSshownumbers

% default values:
\WSshowgridtrue      % show the grid
\WSshownumbersfalse  % do not show the numbering

\let\WSshowgridon\WSshowgridtrue
\let\WSshowgridoff\WSshowgridfalse
\let\WSshownumberson\WSshownumberstrue
\let\WSshownumbersoff\WSshownumbersfalse

\newcommand\WSsetgrid[1]{%
  \csname WSshowgrid#1\endcsname
}
\newcommand\WSsetnumbers[1]{%
  \csname WSshownumbers#1\endcsname
}

% ===== Field dimensions: rows / columns =====
\newcounter{wsrow}   % internal counter
\newcommand\WSrows{0}% total number of rows
\newcommand\WScols{0}% total number of columns

\newcommand\WSreset{%
  \setcounter{wsrow}{0}%
  \gdef\WSrows{0}%
  \gdef\WScols{0}%
}

% #1 = a row of letters without spaces
\newcommand\WSrow[1]{%
  \stepcounter{wsrow}%
  % store the total number of rows
  \xdef\WSrows{\arabic{wsrow}}
  \StrLen{#1}[\rowlen]%
  % store the number of columns from the first row
  \ifnum\value{wsrow}=1\relax
    \xdef\WScols{\rowlen}%
  \fi
  \foreach \c in {1,...,\rowlen}{%
    \StrChar{#1}{\c}[\thischar]%
    \node (m-\arabic{wsrow}-\c) at (\c,-\arabic{wsrow}) {\thischar};%
  }%
}

% style of the box around a word
\tikzset{
  ws box/.style={
    draw=red,
    line width=2pt,
    rounded corners=10pt,
    fill=none
  }
}

% half of the rectangle “thickness”
\newcommand\WSoffset{10pt}
\newcommand\WSgrow{10pt}

%   (r1,c1) and (r2,c2) — first and last letters of a word
%   nodes are named m-<row>-<col>
% #1 — additional TikZ options (for example, draw=..., fill=..., fill opacity=...)
\newcommand\RWord[5][]{%
  % A' and B' – A and B extended by \WSgrow along the word direction
  \path
    coordinate (wsA) at
      ($(m-#2-#3.center)!-\WSgrow!(m-#4-#5.center)$)
    coordinate (wsB) at
      ($(m-#4-#5.center)!-\WSgrow!(m-#2-#3.center)$);
  %
  \draw[ws box,#1]      
    ($(wsA)!\WSoffset!90:(wsB)$)
    --    
    ($(wsA)!-\WSoffset!90:(wsB)$)
    --
    ($(wsB)!\WSoffset!90:(wsA)$)
    --
    ($(wsB)!-\WSoffset!90:(wsA)$)           
    -- cycle;
}


\begin{document}

% ==== rendering mode ====
% debugging (grid + numbering):
\WSsetgrid{off}\WSsetnumbers{off}

\begin{tikzpicture}[
  every node/.style={
    minimum size=6mm,
    font=\ttfamily\large,
  }
]

\WSreset
\WSrow{EHATEUDBLRHSIDS}
\WSrow{RENOUNCEOCWNSNS}
\WSrow{TEPKDRDWOOHRNAU}
\WSrow{DKSCPCJMGNFKOTB}
\WSrow{BWXPFCPYHFUNISM}
\WSrow{VWNYOAQDRETOTRI}
\WSrow{MRGIRNOFGSUDIET}
\WSrow{UITANWSDCSRNDDQ}
\WSrow{CSBOVTPIXREANNS}
\WSrow{VLNJRTBHBQCBOUR}
\WSrow{EYZQPAGYHIEACFO}
\WSrow{XNDMQLHBULLOJLD}
\WSrow{CHOFBYLPMOCIHFU}
\WSrow{QRTENUCREGRETET}
\WSrow{PZIENXGZAQERFYY}


% ----- grid with full frame -----
\ifWSshowgrid
  % internal horizontal lines
  \foreach \r in {1,...,\numexpr\WSrows-1\relax}{%
    \draw[gray!40] (0.5,-\r+0.5) -- (\WScols+0.5,-\r+0.5);
  }
  % internal vertical lines
  \foreach \c in {1,...,\numexpr\WScols-1\relax}{%
    \draw[gray!40] (\c+0.5,-0.5) -- (\c+0.5,-\WSrows-0.5);
  }
  % outer frame
  \draw[gray!60] (0.5,-0.5) rectangle (\WScols+0.5,-\WSrows-0.5);
\fi

% ----- numbering of rows and columns -----
\ifWSshownumbers
  % row numbers on the left
  \foreach \r in {1,...,\WSrows}{%
    \node[anchor=east,font=\scriptsize]
      at (0,-\r) {\r};
  }
  % column numbers on the top
  \foreach \c in {1,...,\WScols}{%
    \node[anchor=south,font=\scriptsize]
      at (\c,-0.1) {\c};
  }
\fi


  \RWord[blue]{1}{2}{1}{6}

  \RWord{1}{2}{7}{2}
  
  \RWord{1}{2}{5}{6}
    
  \RWord[fill=orange,opacity=0.4]{3}{4}{8}{4}

  \RWord[green!80!black,]{4}{3}{15}{14}
  
  \RWord[fill=yellow!40,fill opacity=0.5]{7}{4}{12}{9}
    
  \RWord{12}{1}{12}{9}

  \RWord{13}{1}{13}{10}
  
  \RWord{7}{9}{3}{13}

\end{tikzpicture}
\end{document} 

P.S.
I have added support for filling and for adjusting its transparency. I have also included images that illustrate the remaining three possible combinations of the two switches \WSsetgrid{on/off} and \WSsetnumbers{on/off}.